cdcpb: add CDC scan priority

[hongyunyan]

Background

TiCDC needs to tell downstream CDC services whether an initial scan belongs to a newly created changefeed or to an existing changefeed repair path.

Motivation

Without an explicit priority in ChangeDataRequest, downstream services cannot distinguish low-priority bootstrap scans from high-priority scans that protect existing changefeed lag.

Summary

• Add ScanPriority to cdcpb.
• Add ChangeDataRequest.scan_priority.
• Add ChangeDataRequest.changefeed_id for debugging and observability.
• Regenerate Go bindings and update the proto lock file.
• Merge the latest master and keep the scan-priority protocol fields on top.

Validation

• make check
• make proto-fmt
• make go
• go test ./pkg/cdcpb

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