refactor: move Plugins to plugins.rs and use rwlock

[zyy17]

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What's changed and what's your intention?

It's a minor refactoring for Plugins to make it clean.

In this PR:

1. Move Plugins from lib.rs to the new mod file plugins.rs;
2. Add some simple and necessary unit tests for Plugins;
3. Use RwLock instead of Mutex to improve performance;

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 97.56098% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 85.31%. Comparing base (6e1cc1d) to head (f038057).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3862      +/-   ##
==========================================
- Coverage   85.64%   85.31%   -0.33%     
==========================================
  Files         952      952              
  Lines      163290   163334      +44     
==========================================
- Hits       139843   139351     -492     
- Misses      23447    23983     +536