Issue #2530: Add Autopush support components

[jonalmeida]

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

[jonalmeida]

Some comments from @grigoryk after an offline conversation:

• Don't surface the errors for firebase. This isn't very useful for clients of the push feature to do anything.
• Surface the subscription info to subscribers.
• After some sleuthing in fennec, third party push subscribers only need the FCM registration token in order to do their thing.
  • Remove ThirdParty from PushType.
  • Surface a way for subscribers to read the registration token and listen to token updates.

For AbstractFirebasePushService:

• Re-add forceRegistrationRenewal - this is for Feature Push finding out that the token is invalid on the device.
  • This is hard to do from our abstraction without finding a way to communicate to the service from the Feature bi-directionally.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@87b3747). Click here to learn what that means.
The diff coverage is 88.88%.

@@            Coverage Diff            @@
##             master    #2694   +/-   ##
=========================================
  Coverage          ?   81.41%           
  Complexity        ?     3079           
=========================================
  Files             ?      398           
  Lines             ?    13546           
  Branches          ?     1923           
=========================================
  Hits              ?    11029           
  Misses            ?     1721           
  Partials          ?      796

Impacted Files	Coverage Δ	Complexity Δ
...a/mozilla/components/concept/push/PushProcessor.kt	100% <100%> (ø)	0 <0> (?)
...s/lib/push/firebase/AbstractFirebasePushService.kt	77.77% <77.77%> (ø)	4 <4> (?)

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[grigoryk]

(first pass)

[grigoryk]

I think we can kill some complexity and abstraction layers here pretty easily. Specifically, the bus and the RustPushConnection class. It's fine if you'd want to do this in a follow-up, just make sure to file issues and outline the cleanup work in comments.

[grigoryk]

You've certainly thought enough about this - let's land it, and iterate in the tree.

Great work!! 💯