tools: add a CLI for keyspace GC barriers

[wfxr]

What problem does this PR solve?

A stale TiCDC GC barrier can prevent obsolete data from being reclaimed. Operators need a way to inspect, update, and delete individual barriers so they can control how much history becomes eligible for GC.

This is a review-only draft and is not intended to be merged. The implementation is based on v8.5.4-nextgen.202510.7 and uses that version's existing PD gRPC APIs.

Issue Number: None (review-only draft).

What is changed and how does it work?

Add a standalone pd-gc-barrier CLI with show, set, and delete commands for explicitly selected keyspaces. Set creates or updates both the barrier timestamp and TTL, including non-expiring barriers.

Accept decimal TSO and timezone-bearing RFC3339 input, validate the requested GC scope, and preserve full TSO precision in JSON output. Support TLS credentials, command timeouts, and clear error reporting. Include unit tests, local NextGen PD integration coverage, and an English usage reference.

Check List

Tests

• Unit tests and local PD integration test: go test -tags nextgen,without_dashboard,integration ./pd-gc-barrier -count=1 -timeout=5m from tools/, with failpoints enabled and then disabled.
• Static checks: targeted golangci-lint passed for the tool and its tests.
• Manual executable checks: help, invalid keyspace IDs, TSO/TTL validation, TLS argument validation, connection timeout, nonzero error exit status, and stderr/stdout separation.

Release note

None.

[ti-chi-bot]

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[wfxr]

/hold

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[codecov]

Codecov Report

❌ Patch coverage is 76.39752% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.66%. Comparing base (14fe15c) to head (e7eabf7).
⚠️ Report is 9 commits behind head on release-nextgen-20251011.

Additional details and impacted files

@@                     Coverage Diff                      @@
##           release-nextgen-20251011   #11223      +/-   ##
============================================================
- Coverage                     78.73%   78.66%   -0.07%     
============================================================
  Files                           491      492       +1     
  Lines                         66124    66498     +374     
============================================================
+ Hits                          52060    52313     +253     
- Misses                        10308    10425     +117     
- Partials                       3756     3760       +4     

Flag	Coverage Δ
unittests	78.66% <76.39%> (-0.07%)	⬇️

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