Support overriding third party methods ; Support adding new methods to third party structs

[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 96.61017% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.41%. Comparing base (2db59f2) to head (04b0ea5).
Report is 32 commits behind head on master.

Files	Patch %	Lines
frb_macros/src/components/attr_external.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2029      +/-   ##
==========================================
+ Coverage   97.41%   98.41%   +0.99%     
==========================================
  Files         419      419              
  Lines       17047    17133      +86     
==========================================
+ Hits        16606    16861     +255     
+ Misses        441      272     -169     

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