Initial commit for the relevancy component

[bendk]

Added boilerplate for UniFFI, error handling, databases, etc. I tried to follow the best practices from other application-services components.

Created the initial types and methods for the API. I think all of these should be considered works-in-progress. It seems likely that some or all of them will change as we implement the functionality.

Added some code to read / write URL interest data from a test binary file currently stored in the repo. This is also a WIP, at the very least it needs to change to download the data from remote settings.

Pull Request checklist

• 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
  • Note:
    • For changes that need extra cross-platform testing, consider adding [ci full] to the PR title.
    • If this pull request includes a breaking change, consider cutting a new release after merging.
• [ ] Tests: This PR includes thorough tests or an explanation of why it does not
• [ ] Changelog: This PR includes a changelog entry in CHANGELOG.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.

Branch builds: add [firefox-android: branch-name] to the PR title.

[bendk]

Created the initial types and methods for the API. I think all of these should be considered works-in-progress. It seems likely that some or all of them will change as we implement the functionality.

So, I think this review should focus on "does this code seem good enough to start with?" rather than "is it correct?"

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.08%. Comparing base (78f2d58) to head (2a34bc6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6152   +/-   ##
=======================================
  Coverage   84.08%   84.08%           
=======================================
  Files         117      117           
  Lines       15629    15629           
=======================================
  Hits        13141    13141           
  Misses       2488     2488           

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