Validating buf.validate rules on buffa types

[zs-dima]

If you annotate your protos with buf.validate rules, there are now two ways to enforce them on buffa types:

• prost-protovalidate — one library for prost and buffa; standard rules compile to direct field-access validators, CEL via the bundled runtime engine
• protovalidate-buffa — buffa-native, CEL transpiled to Rust

Both pass the upstream conformance suite. prost-protovalidate v0.6.0 release notes: