Starting new Remote Settings API

[bendk]

Implemented client functionality and started the general structure for the types. The big item left todo is storage.

Added a CLI to test it, you can run it using cargo rs.

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• Breaking changes: This PR follows our breaking change policy
  • [ ] This PR follows the breaking change policy:
    • This PR has no breaking API changes, or
    • There are corresponding PRs for our consumer applications that resolve the breaking changes and have been approved
• [ ] Quality: This PR builds and tests run cleanly
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• [ ] Tests: This PR includes thorough tests or an explanation of why it does not
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Branch builds: add [firefox-android: branch-name] to the PR title.

[bendk]

This new API is still a WIP, but I think we still could merge this since no consumers are using it yet. I tested it with the cargo rs CLI and it seemed to work.

@leplatrem I added you as a reviewer mostly to make sure that the client code is hitting the right endpoints and following the client specs correctly.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 226 lines in your changes missing coverage. Please review.

Project coverage is 21.82%. Comparing base (50bf235) to head (0da86bf). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
components/remote_settings/src/client.rs	0.00%	106 Missing :warning:
components/remote_settings/src/service.rs	0.00%	54 Missing :warning:
components/remote_settings/src/lib.rs	0.00%	41 Missing :warning:
components/remote_settings/src/storage.rs	0.00%	24 Missing :warning:
components/remote_settings/src/config.rs	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6417      +/-   ##
==========================================
- Coverage   21.98%   21.82%   -0.17%     
==========================================
  Files         342      344       +2     
  Lines       30737    30971     +234     
==========================================
  Hits         6759     6759              
- Misses      23978    24212     +234     

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