refactor: optimize the usage of comment widget extension point

[guqing]

What type of PR is this?

/kind improvement
/area core
/milestone 2.8.x

What this PR does / why we need it:

优化评论扩展点的使用方式

how to test it?
测试评论插件是否正常可用

Does this PR introduce a user-facing change?

优化评论扩展点的使用方式

[codecov]

Codecov Report

Merging #4249 (ad469fe) into main (5a0e202) will increase coverage by 0.00%.
The diff coverage is 90.00%.

@@            Coverage Diff            @@
##               main    #4249   +/-   ##
=========================================
  Coverage     60.14%   60.14%           
- Complexity     2413     2415    +2     
=========================================
  Files           366      366           
  Lines         12587    12593    +6     
  Branches        906      907    +1     
=========================================
+ Hits           7570     7574    +4     
- Misses         4578     4579    +1     
- Partials        439      440    +1     

Impacted Files	Coverage Δ
.../plugin/extensionpoint/DefaultExtensionGetter.java	60.00% <84.61%> (-0.98%)	⬇️
.../app/theme/dialect/CommentElementTagProcessor.java	100.00% <100.00%> (ø)

[JohnNiang]

/lgtm

[f2c-ci-robot]

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