api/etcdserverpb/rpc.proto: add leader_id response header

[gyuho]

(/cc @SaranBalaji90 @jamesmthompson)

Adds leader_id (field 5) to the v3 ResponseHeader and fills it at every server-side header site from the already-wired RaftStatusGetter.Leader(). The field is optional: 0 means "no hint," and it is not a fencing token. Old clients ignore it; new clients treat 0 from old servers as "unknown leader." Cost is at most ~11 bytes per response for a value the server already holds in memory.

Today a client must poll Status to learn the leader; this field puts that leader member ID in every Put/Get/Watch response, which is what Phase 2's opt-in leader-aware balancer builds on.

A new common-suite test (TestResponseHeaderLeaderId, integration and e2e) verifies leader_id is nonzero once a leader is elected, matches each member's status.leader, agrees across members, and appears on Put and Get, with mixed-version clusters checked per current-version endpoint.

c.f., #22268

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[kubernetes-prow]

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[fuweid]

/ok-to-test

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.72%. Comparing base (23a4e40) to head (575d780).

Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/apply/backend.go	86.11% <100.00%> (+0.07%)	⬆️

... and 25 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #22327      +/-   ##
==========================================
- Coverage   69.76%   69.72%   -0.05%     
==========================================
  Files         448      448              
  Lines       38159    38164       +5     
==========================================
- Hits        26622    26610      -12     
- Misses      10107    10122      +15     
- Partials     1430     1432       +2     

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