proto: add ExplainForConnection executor

[panmuyun]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

Cluster-level EXPLAIN FOR CONNECTION needs to resolve and execute the request on a remote TiDB node when the target connection does not belong to the current server. The current tipb executor protocol does not have a dedicated executor type for this operation, so TiDB cannot encode the target connection_id and dispatch the request through cop RPC in a structured way.

What is changed and how it works?

What's Changed:

• Add a new executor type TypeExplainForConnection to ExecType
• Add a new ExplainForConnection protobuf message with connection_id
• Add explain_for_connection field to Executor
• Regenerate go-tipb/executor.pb.go from the updated protobuf definition

How it Works:

• The protobuf schema introduces ExplainForConnection as a dedicated executor payload.
• Executor.tp can now be set to TypeExplainForConnection.
• When this executor is used, the target connection is identified by Executor.explain_for_connection.connection_id.
• The regenerated Go binding exposes the new enum value, struct, accessor, marshal/unmarshal, and registration code for downstream consumers.

Proposal: xxx

What's Changed:

How it Works:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

[pingcap-cla-assistant]

All committers have signed the CLA.