[WIP] Initialize confState from v3 store on bootstrap

[ahrtr]

Main changes:

• build confState from v3store
• don't apply confChange to raft if it's already applied, so that we can avoid the issue mentioned in Remove v2 store #20139 (comment)

Note this PR depends on etcd-io/raft#370, so we need to merge the raft PR first.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[k8s-ci-robot]

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.42%. Comparing base (5931270) to head (fb21a52).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
server/etcdserver/bootstrap.go	79.06%	4 Missing and 5 partials ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/server.go	83.38% <100.00%> (+0.08%)	⬆️
server/etcdserver/bootstrap.go	67.01% <79.06%> (+1.19%)	⬆️

... and 18 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21138      +/-   ##
==========================================
+ Coverage   68.29%   68.42%   +0.13%     
==========================================
  Files         429      429              
  Lines       35242    35286      +44     
==========================================
+ Hits        24067    24145      +78     
+ Misses       9775     9743      -32     
+ Partials     1400     1398       -2     

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