VTGate StreamExecute rpc to return session as response

[harshit-gangal]

Description

This PR fixes the issue with StreamExecute VTGate RPC response. It adds the VTGate Session to the response.
This fixes all the side effects caused by not returning the modified session.

The Session is sent as the last message on the stream after all the Query Results are sent. So, the last stream message will have QueryResult as nil and Session as not nil.
Any error that happened during the query phase will be sent only after sending the Session.

Note: This is not an issue if MySQL clients are used to connect to VTGate.

Related Issue(s)

• Fixes VTGate StreamExecute grpc api does not support transactions #13132

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[maxenglander]

Is this still true?

[harshit-gangal]

	// StreamExecute is only used for SELECT queries that don't change
	// the session. So, the protocol doesn't return an updated session.
	// This may change in the future.

For sure it has changed and this is the future :)

[GuptaManan100]

LGTM