feat(cmd/protoc-gen-go-http): add comment to http server interface method

[thinkgos]

Description (what this PR does / why we need it):

add comment to http server interface, we can find the method use.

Which issue(s) this PR fixes (resolves / be part of):

easy to find what is the http server method use.

Other special notes for the reviewers:

nothing

[codecov-commenter]

Codecov Report

Merging #2657 (4d50ddb) into main (7def38a) will not change coverage.
The diff coverage is n/a.

❗ Current head 4d50ddb differs from pull request most recent head 509551b. Consider uploading reports for the commit 509551b to get more accurate results

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@@           Coverage Diff           @@
##             main    #2657   +/-   ##
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  Coverage   84.89%   84.89%           
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  Files          88       88           
  Lines        3873     3873           
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  Hits         3288     3288           
  Misses        411      411           
  Partials      174      174           

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[shenqidebaozi]

LGTM