Nimbus-SDK-119 Get Nimbus server endpoint at build time.

[jhugman]

This PR builds on AC##9024, and implements setting the Nimbus endpoints from a secret set at build time.

For production use, this requires a secret named nimbus_url to be put into CI.

Note: Nimbus is currently behind a feature flag.

If developers wish to use a Nimbus server for local development, you can set the url by adding an entry into local.properties, e.g.:

nimbus.remote-settings.url=https://settings.stage.moz4ws.net

Without setting server, Nimbus will be able to function, except no experimental definitions will be fetched, and features under experiment will be configured as if not enrolled in the experiment.

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[mergify]

This pull request has conflicts when rebasing. Could you fix it @jhugman? 🙏

[pocmo]

FIY: We just landed the AC update on master (with server=null to work around the breaking API change)

[codecov-io]

Codecov Report

Merging #16682 (590db8b) into master (c465521) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master   #16682      +/-   ##
============================================
- Coverage     30.26%   30.25%   -0.01%     
  Complexity     1231     1231              
============================================
  Files           455      455              
  Lines         18654    18658       +4     
  Branches       2577     2578       +1     
============================================
  Hits           5645     5645              
- Misses        12562    12566       +4     
  Partials        447      447              

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/org/mozilla/fenix/components/Analytics.kt	52.45% <0.00%> (-3.69%)	1.00 <0.00> (ø)

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[jhugman]

[task 2020-11-20T16:51:44.876Z] + taskcluster/scripts/get-secret.py -s project/mobile/fenix/nightly-simulation -k nimbus_url -f .nimbus
[task 2020-11-20T16:51:45.151Z] Outputting secret to: /builds/worker/checkouts/src/.nimbus
[task 2020-11-20T16:51:45.151Z] Traceback (most recent call last):
[task 2020-11-20T16:51:45.151Z]   File "taskcluster/scripts/get-secret.py", line 69, in <module>
[task 2020-11-20T16:51:45.151Z]     main()
[task 2020-11-20T16:51:45.151Z]   File "taskcluster/scripts/get-secret.py", line 65, in main
[task 2020-11-20T16:51:45.151Z]     write_secret_to_file(result.path, secret, result.key, result.decode, result.json, result.append, result.prefix)
[task 2020-11-20T16:51:45.151Z]   File "taskcluster/scripts/get-secret.py", line 27, in write_secret_to_file
[task 2020-11-20T16:51:45.151Z]     value = data['secret'][key]
[task 2020-11-20T16:51:45.151Z] KeyError: 'nimbus_url'

This looks like it requires a secret URL put on taskcluster. Who would be the right person to convey this secret to to get this on to taskcluster? @pocmo ? @amedyne ? @st3fan ?

[pocmo]

@JohanLorenzo or @MihaiTabara can help with that, I think. What URL do we need to use (or is that actually secret?)? I may have access too, but I am not sure if I remember the right paths for all build variants on https://firefox-ci-tc.services.mozilla.com/secrets