Add access to the descriptor (of a protobuf message).

[yangzh]

Feature description:

would it be possible to add access (from CEL) to the descriptor of a message (or a field, an enum, etc).

Problem it solves or use case:
I would like to store the schema / descriptor of a protobuf message for a BLOB column (in database, for example), and it would be helpful if I can have a self-contained descriptor with the column via CEL.

Proposed implementation or solution:

This can be in the form of a new CEL function, like this:

descriptor(this) -> DescriptorProto

Contribution:
No, but I'm happy to test / feedback.

[yangzh]

I happened to notice this previous issue (#320)