Rewrite Nimbus.kt and FxaClient in terms of a decorator

[jhugman]

Draft.

This is an implementation of Nimbus.kt with the proposed Uniffi decorators.

Many Nimbus method calls go through the NimbusClientDecorator, and almost all the methods which were forwarding to the NimbusClient have now disappeared.

It is for illustrative and exploratory purposes only at this time.

I will self-review to highlight what I learned in the process of doing it.

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• [ ] Quality: This PR builds and tests run cleanly
  • Note:
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• [ ] Tests: This PR includes thorough tests or an explanation of why it does not
• [ ] Changelog: This PR includes a changelog entry in CHANGES_UNRELEASED.md or an explanation of why it does not need one
  • Any breaking changes to Swift or Kotlin binding APIs are noted explicitly
• [ ] Dependencies: This PR follows our dependency management guidelines
  • Any new dependencies are accompanied by a summary of the due dilligence applied in selecting them.

[codecov-commenter]

Codecov Report

Merging #4595 (6ca5b26) into main (51009b4) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4595   +/-   ##
=======================================
  Coverage   80.69%   80.69%           
=======================================
  Files          48       48           
  Lines        5217     5217           
=======================================
  Hits         4210     4210           
  Misses       1007     1007           

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[jhugman]

Closing until further discussion is had.