feat: support apiEndpoint override in client constructor

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:846102ebf7ea2239162deea69f64940443b4147f7c2e68d64b332416f74211ba
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/datastore/artman_datastore.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/js/datastore-v1.
synthtool > Replaced '../../package.json' in src/v1/datastore_client.js.
.eslintignore
.eslintrc.yml
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.jsdoc.js
.kokoro/common.cfg
.kokoro/continuous/node10/common.cfg
.kokoro/continuous/node10/docs.cfg
.kokoro/continuous/node10/lint.cfg
.kokoro/continuous/node10/samples-test.cfg
.kokoro/continuous/node10/system-test.cfg
.kokoro/continuous/node10/test.cfg
.kokoro/continuous/node11/common.cfg
.kokoro/continuous/node11/test.cfg
.kokoro/continuous/node8/common.cfg
.kokoro/continuous/node8/test.cfg
.kokoro/docs.sh
.kokoro/lint.sh
.kokoro/presubmit/node10/common.cfg
.kokoro/presubmit/node10/docs.cfg
.kokoro/presubmit/node10/lint.cfg
.kokoro/presubmit/node10/samples-test.cfg
.kokoro/presubmit/node10/system-test.cfg
.kokoro/presubmit/node10/test.cfg
.kokoro/presubmit/node11/common.cfg
.kokoro/presubmit/node11/test.cfg
.kokoro/presubmit/node8/common.cfg
.kokoro/presubmit/node8/test.cfg
.kokoro/presubmit/windows/common.cfg
.kokoro/presubmit/windows/test.cfg
.kokoro/publish.sh
.kokoro/release/docs.cfg
.kokoro/release/docs.sh
.kokoro/release/publish.cfg
.kokoro/samples-test.sh
.kokoro/system-test.sh
.kokoro/test.bat
.kokoro/test.sh
.kokoro/trampoline.sh
.nycrc
.prettierignore
.prettierrc
CODE_OF_CONDUCT.md
CONTRIBUTING.md
LICENSE
README.md
codecov.yaml
renovate.json
samples/README.md

> core-js@2.6.9 postinstall /tmpfs/src/git/autosynth/working_repo/node_modules/core-js
> node scripts/postinstall || echo "ignore"

Thank you for using core-js ( https://github.com/zloirock/core-js ) for polyfilling JavaScript standard library!

The project needs your help! Please consider supporting of core-js on Open Collective or Patreon: 
> https://opencollective.com/core-js 
> https://www.patreon.com/zloirock 

Also, the author of core-js ( https://github.com/zloirock ) is looking for a good job -)


> protobufjs@6.8.8 postinstall /tmpfs/src/git/autosynth/working_repo/node_modules/protobufjs
> node scripts/postinstall


> @google-cloud/datastore@4.0.0 prepare /tmpfs/src/git/autosynth/working_repo
> npm run compile


> @google-cloud/datastore@4.0.0 compile /tmpfs/src/git/autosynth/working_repo
> tsc -p . && cp -r src/v1 build/src && cp -r proto* build && cp test/*.js build/test

npm notice created a lockfile as package-lock.json. You should commit this file.
added 694 packages from 1237 contributors and audited 2113 packages in 21.839s
found 0 vulnerabilities


> @google-cloud/datastore@4.0.0 fix /tmpfs/src/git/autosynth/working_repo
> gts fix && eslint '**/*.js' --fix


/tmpfs/src/git/autosynth/working_repo/samples/addTask.js
  19:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/concepts.js
  25:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/quickstart.js
  20:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/system-test/concepts.test.js
  20:23  error  "@google-cloud/nodejs-repo-tools" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/system-test/quickstart.test.js
  21:23  error  "@google-cloud/nodejs-repo-tools" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/system-test/tasks.test.js
  18:29  error  "@google-cloud/datastore" is not found          node/no-missing-require
  22:23  error  "@google-cloud/nodejs-repo-tools" is not found  node/no-missing-require

/tmpfs/src/git/autosynth/working_repo/samples/tasks.js
  23:29  error  "@google-cloud/datastore" is not found  node/no-missing-require

✖ 8 problems (8 errors, 0 warnings)

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @google-cloud/datastore@4.0.0 fix: `gts fix && eslint '**/*.js' --fix`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the @google-cloud/datastore@4.0.0 fix script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/kbuilder/.npm/_logs/2019-06-04T19_30_55_835Z-debug.log
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[codecov]

Codecov Report

Merging #422 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #422   +/-   ##
=======================================
  Coverage   98.05%   98.05%           
=======================================
  Files           5        5           
  Lines         617      617           
  Branches      145      145           
=======================================
  Hits          605      605           
  Partials       12       12

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