Export more objects to allow custom thread pool without using flutter_rust_bridge::for_generated namespace

[fzyzcjy]

Changes

Close #2215

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.06%. Comparing base (78ce4f7) to head (53d83c7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2228   +/-   ##
=======================================
  Coverage   99.06%   99.06%           
=======================================
  Files         488      488           
  Lines       20184    20184           
=======================================
  Hits        19995    19995           
  Misses        189      189           

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