Support #[frb(proxy)] (part 1)

[fzyzcjy]

Changes

Please list issues fixed by this PR here, using format "Fixes #the-issue-number".

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

Attention: Patch coverage is 96.85864% with 12 lines in your changes missing coverage. Please review.

Project coverage is 95.26%. Comparing base (5f2065e) to head (7bcc32f).

Files	Patch %	Lines
.../codegen/parser/mir/parser/function/real/output.rs	89.28%	3 Missing ⚠️
frb_dart/lib/src/misc/simple_disposable.dart	0.00%	3 Missing ⚠️
.../generator/api_dart/spec_generator/class/method.rs	95.55%	2 Missing ⚠️
...st/spec_generator/codec/cst/decoder/ty/delegate.rs	0.00%	2 Missing ⚠️
...rt/spec_generator/codec/cst/encoder/ty/delegate.rs	0.00%	1 Missing ⚠️
...odegen/parser/mir/parser/function/real/argument.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2050      +/-   ##
==========================================
- Coverage   96.95%   95.26%   -1.70%     
==========================================
  Files         460      458       -2     
  Lines       18093    16899    -1194     
==========================================
- Hits        17542    16098    -1444     
- Misses        551      801     +250     

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