feat(kafka): allow protobuf schema

[Serpentiel]

About this change—what it does

allows usage of protobuf schema in Kafka resources

resolves #1366

Why this way

we would want to have this feature in place

[ivan-savciuc]

LGTM, the only proposition of my would be to use a go-protobuf library for validation and normalization

[Serpentiel]

I don't see any reason having such a validation. It basically checks that string contains syntax = proto3; substring. Which is not a valid syntax btw.

so what do you suggest changing it with then? I checked, it works just fine as long as the string looks like syntax...=...proto3...;

we need some kind of validation in this place to be able to combine it together with the existing JSON validation mechanic

[Serpentiel]

@byashimov I've updated the PR with the fix

[byashimov]

lgtm

not relevant