[DO NOT MERGE] Expose webext-storage bridged engine logic

[lougeniaC64]

Exposing the webext-storage bridged engine work for integration of the uniffi-ed components in desktop. This PR will not be merged until the desktop patch(es) to refactor the webext-storage logic and vendor in A-S are reviewed and approved.

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 22.64%. Comparing base (46acbeb) to head (5244fb8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6180   +/-   ##
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  Coverage   22.64%   22.64%           
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  Files         330      330           
  Lines       29811    29811           
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  Hits         6750     6750           
  Misses      23061    23061           

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[mhammond]

This looks great - just a few nits. Obviously we still need to be careful about when exactly we land this, but it looks ready to me, nice work!

[lougeniaC64]

@linabutler @skhamis I'm adding you as reviewers to give you a chance to review these changes and offer any feedback.

[linabutler]

Thanks so much, @lougeniaC64! This looks super reasonable to me—I see it matches TabsBridgedEngine concept-for-concept, which is great!

I don't think there's any hurry to consolidate them, especially since it would be nice to eventually bridge the other way—that is, use our Rust Sync code in Desktop, and have Desktop-specific engines implemented in JS—so I'm completely happy to approve this once it's out of draft!