[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.

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

UPDATE: From thread https://bugzilla.mozilla.org/show_bug.cgi?id=1497430#c12.
This is how it works.

1. This allows client code to get the supported encodings from the browser
2. The client also requests a subscription, which comes from the push manager (not the app server)
3. The app server needs this information from the client in order to know the push endpoint, what keys to use to encrypt the content, and the algorithm. The spec does not state how it gets it, so we have to define our own mechanism. One way would be to bundle with the subscription. https://web.dev/articles/push-notifications-subscribing-a-user#send_a_subscription_to_your_server

Status

• BCD - Updates for Firefox 134 beta browser-compat-data#25230
• Docs = PushManager.supportedContentEncodings updates #36962
• Release note FF134 PushManager.supportedContentEncodings() supported #36975
• dev-docs-complete