[WebAPI] Implement PushManager.supportedContentEncodings

[Rumyra]

Acceptance criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes for enabled/preview features
• Add/remove entry to Firefox experimental features page for preference/released features

Related Gecko bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1497430

Other

• Check content open issues to see if any pertain to the subject matter. If there are any that can be closed because of the work, do so. If there are any that can be fixed relatively quickly because of the knowledge from completing this issue and you have time, feel free to go ahead and fix them.

[hamishwillee]

This is shipped (not just implemented) and in web workers. There are some "nuances" to the implementation but they are not externally visible and so irrelevant.

I understand that this allows client code to get the algorithm used for encrypting push messages. I don't understand how this gets to the application server. Still chasing that: https://bugzilla.mozilla.org/show_bug.cgi?id=1497430#c12

FYI WPT tests here https://wpt.live/push-api/supported-encodings.https.any.html

Status

• BCD - Waiting on BCD team.
• Docs = PushManager.supportedContentEncodings updates #36962
• Release note FF134 PushManager.supportedContentEncodings() supported #36975
• dev-docs-complete