optimize: support HTTP/2 push for the Watch API in Server Raft mode

[YvCeung]

• [x] I have read the CONTRIBUTING.md guidelines.
• [x] I have registered the PR changes.

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov[bot]]

Codecov Report

:x: Patch coverage is 80.64516% with 6 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 67.30%. Comparing base (61e375f) to head (19314b1). :warning: Report is 1 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
.../server/cluster/manager/ClusterWatcherManager.java	80.64%	2 Missing and 4 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7752      +/-   ##
============================================
+ Coverage     66.17%   67.30%   +1.12%     
+ Complexity      994      992       -2     
============================================
  Files          1323     1323              
  Lines         50155    50173      +18     
  Branches       5928     5934       +6     
============================================
+ Hits          33190    33768     +578     
+ Misses        14053    13480     -573     
- Partials       2912     2925      +13     

Files with missing lines	Coverage Δ
.../server/cluster/manager/ClusterWatcherManager.java	79.71% <80.64%> (+5.20%)	:arrow_up:

... and 40 files with indirect coverage changes

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