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

[YvCeung]

• I have read the CONTRIBUTING.md guidelines.
• 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

[funky-eyes]

I want to keep this connection open and reuse it to deliver updates whenever the leader changes, rather than requiring the client to open a new HTTP/2 watch request after every response.

[YvCeung]

done in 45f362e

[codecov]

Codecov Report

❌ Patch coverage is 60.97561% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.16%. Comparing base (61e375f) to head (45f362e).

Files with missing lines	Patch %	Lines
.../server/cluster/manager/ClusterWatcherManager.java	60.97%	11 Missing and 5 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7752      +/-   ##
============================================
- Coverage     66.17%   66.16%   -0.02%     
+ Complexity      994      992       -2     
============================================
  Files          1323     1323              
  Lines         50155    50184      +29     
  Branches       5928     5935       +7     
============================================
+ Hits          33190    33203      +13     
- Misses        14053    14063      +10     
- Partials       2912     2918       +6     

Files with missing lines	Coverage Δ
.../server/cluster/manager/ClusterWatcherManager.java	70.00% <60.97%> (-4.51%)	⬇️

... and 7 files with indirect coverage changes

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