kvrpcpb: add delete flag to RawCASRequest to enable compare-and-delete

[healthwaite]

Add a delete flag into the RawCASRequest message. This allows us to extend the CompareAndSwap API to implement compare and delete semantics as discussed in this issue. When delete is set CompareAndSwap will delete the key if cur_val==expected_val.

Accompanying tikv PR here

[pingcap-cla-assistant]

All committers have signed the CLA.

[healthwaite]

Is it possible anyone can take a look at reviewing this please? Possiby @cfzjywxk? The accompanying tikv PR has been reviewed and approved.

[glorv]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: glorv
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-02-25 02:25:30.567028781 +0000 UTC m=+235403.081823390: ☑️ agreed by glorv.

[healthwaite]

Any chance you can take a look at this when you have a second please @cfzjywxk ?