Allow selecting a protobuf .proto or .desc schema when parsing protobuf data

[pimterry]

We need to:

• Provide a UI to select these files, if one isn't specified (should this live in the Pro settings, mapping URLs to schemas? Or be selected from a header in the body section directly?)
• Allow selecting and/or auto-detecting (from content-type headers) the message type, as an inline option on the message content (or just test against all message types, if there aren't too many?)
• Auto-detect desc URLs if included in the content-type and offer to fetch them

Supporting both proto & desc files isn't necessarily required, but it might play nicer with autogeneration of .proto files for reverse engineering work if we can handle both, and it's just generally more convenient.

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