feat: support apiEndpoint override

[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:ce425884865f57f18307e597bca1a74a3619b7098688d4995261f3ffb3488681
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/pubsub/artman_pubsub.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/js/pubsub-v1.
.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
synthtool > Replaced '  }\n\\s*/\\*\\*\n\\s+\\* The DNS address for this API service\\.' in src/v1/subscriber_client.js.
synthtool > Replaced '../../package.json' in src/v1/publisher_client.js.
synthtool > Replaced '../../package.json' in src/v1/subscriber_client.js.
synthtool > Replaced 'https:\\/\\/cloud\\.google\\.com[\\s\\*]*http:\\/\\/(.*)[\\s\\*]*\\)' in src/v1/doc/google/protobuf/doc_timestamp.js.
synthtool > No replacements made in src/**/doc/google/protobuf/doc_timestamp.js for pattern toISOString\], maybe replacement is not longer needed?

> grpc@1.21.1 install /tmpfs/src/git/autosynth/working_repo/node_modules/grpc
> node-pre-gyp install --fallback-to-build --library=static_library

node-pre-gyp WARN Using needle for node-pre-gyp https download 
[grpc] Success: "/tmpfs/src/git/autosynth/working_repo/node_modules/grpc/src/node/extension_binary/node-v57-linux-x64-glibc/grpc_node.node" is installed via remote

> 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/pubsub@0.29.1 prepare /tmpfs/src/git/autosynth/working_repo
> npm run compile


> @google-cloud/pubsub@0.29.1 compile /tmpfs/src/git/autosynth/working_repo
> tsc -p . && cp -r src/v1 build/src/ && cp -r protos build/ && cp -r proto build/ && cp smoke-test/*.js build/system-test && cp test/*.js build/test

npm notice created a lockfile as package-lock.json. You should commit this file.
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@1.2.9 (node_modules/fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.2.9: wanted {"os":"darwin","arch":"any"} (current: {"os":"linux","arch":"x64"})

added 1090 packages from 1362 contributors and audited 6255 packages in 33.383s
found 0 vulnerabilities


> @google-cloud/pubsub@0.29.1 fix /tmpfs/src/git/autosynth/working_repo
> eslint --fix '**/*.js' && gts fix

synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[codecov]

Codecov Report

Merging #647 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #647   +/-   ##
=======================================
  Coverage   97.89%   97.89%           
=======================================
  Files          14       14           
  Lines         856      856           
  Branches      178      178           
=======================================
  Hits          838      838           
  Misses          2        2           
  Partials       16       16

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