feat: add serverCertificateUri in DRM advanced config

[valotvince]

Description

Fixes #1906

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update

Checklist:

• I have signed the Google CLA https://cla.developers.google.com
• My code follows the style guidelines of this project
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have verified my change on multiple browsers on different platforms
• I have run ./build/all.py and the build passes
• I have run ./build/test.py and all tests pass

Works when running alone but doesn't when running with all the other tests (same thing on master)

Chrome 89.0.4389.128 (Mac OS 10.15.7) Offline stores, plays, and deletes protected content with a persistent license FAILED

[TheModMaker]

This isn't really needed as the CDM will request it for us anyway. I'd suggest dropping the demo changes to make things simpler.

[valotvince]

Yep, it was just to have a working example with the serverCertificateUri. I could drop it or comment to explain this shouldn't be needed, but it is here to demonstrate the feature, WYT ?

[valotvince]

@TheModMaker 👋 I've pushed some edits base on your review, thanks !

[shaka-bot]

Test Failure:

Generating Closure dependencies...
Linting JavaScript...

/var/lib/jenkins/workspace/Manual PR Test (local-tests)/test/media/drm_engine_unit.js
1040:1  error  Expected indentation of 10 spaces but found 8  indent
1041:1  error  Expected indentation of 10 spaces but found 8  indent

✖ 2 problems (2 errors, 0 warnings)
2 errors and 0 warnings potentially fixable with the `--fix` option.

END-BUILD: FAILURE
Build step 'Execute shell' marked build as failure

[shaka-bot]

All tests passed!