Support TiDB smooth upgrade in v2

[tennix]

What is changed

This PR adds TiDB smooth-upgrade orchestration to tidb-operator v2:

• extend the v2 TiDB API client with bodyless POST /upgrade/start and POST /upgrade/finish
• call /upgrade/start before switch-controlled TiDBGroup version rollouts mutate TiDB instance CRs
• persist active pause state in controller-owned TiDBGroup annotations
• recover stale/mismatched annotations by calling idempotent /upgrade/finish before clearing them
• call /upgrade/finish after all TiDB instances are on the target version/revision and Ready
• add an RFC for the v2 design

Notes

The behavior follows the v1 smooth-upgrade support from #6904, adapted to the v2 TiDBGroup/TiDB instance reconciliation model.

Tests

PASS go test ./pkg/tidbapi/v1 ./pkg/controllers/tidb ./pkg/controllers/tidb/tasks ./pkg/controllers/tidbgroup ./pkg/controllers/tidbgroup/tasks -count=1
PASS go test ./pkg/controllers/... -count=1
PASS git diff --check

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.58333% with 61 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.64%. Comparing base (6941259) to head (71ae347).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6933      +/-   ##
==========================================
+ Coverage   39.30%   39.64%   +0.34%     
==========================================
  Files         430      431       +1     
  Lines       24310    24550     +240     
==========================================
+ Hits         9554     9733     +179     
- Misses      14756    14817      +61     

Flag	Coverage Δ
unittest	39.64% <74.58%> (+0.34%)	⬆️

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