Use Raw MySQL packets when passing results back from VTTablet to VTGate

[mattlord]

Description

When the VTgate supports it, this eliminates the costly and unnecessary serialization from the MySQL protocol to protobuf and back again, instead passing the raw MySQL packet in the gRPC message as bytes. This is important as while a query is typically small in bytes, the results are often large and all of this unnecessary copying and allocation cost a lot while providing nothing of value.

Related Issue(s)

• [wip] Raw MySQL packets #15522

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

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Backward compatibility

• Protobuf changes should be wire-compatible.
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[harshit-gangal]

This could be giving away hot row/shard protection.