rfc: add built-in "protobuf" data type support

[XiaochenCui]

With the built-in "protobuf" type, the protobuf data which is stored in the database can be rendered as a human-readable format directly. It can also be queried and indexed directly.

The original issue is #47534.

Fixes: None

Release note: None

[blathers-crl]

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[cockroach-teamcity]

This change is 

[blathers-crl]

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[XiaochenCui]

@dt I'm working on a "protobuf" type proposal, this may simplifier queries with protobuf data.

[perezd]

Have you considered proposing a schema registry for managing Protobuf descriptor data requirements within CDB? Our product offers a reflection API for hosted Protobuf modules that would be a very convenient way of delivering and managing the raw descriptor layouts for this RFC: https://buf.build/docs/bsr/introduction

If you're interested, I'd be happy to chat about it in Slack (I'm in the CDB community Slack).

[XiaochenCui]

Have you considered proposing a schema registry for managing Protobuf descriptor data requirements within CDB? Our product offers a reflection API for hosted Protobuf modules that would be a very convenient way of delivering and managing the raw descriptor layouts for this RFC: https://buf.build/docs/bsr/introduction

If you're interested, I'd be happy to chat about it in Slack (I'm in the CDB community Slack).

Thanks for your information, I'll do some research in the next week and come back to you.