Does libserdes support for protobuf schema

I defined a protobuf schema on confluent schema registry. when I use serdes_schema_get function to get a schema, there is a error message.
```
Failed to get schema: Error parsing JSON: invalid token near 'syntax'
```

Start at serdes_schema_get and trace how schema-registry responses are parsed, using the reported protobuf input and the existing Avro-oriented behavior as context. Done means the project’s protobuf-schema compatibility is resolved and covered by an appropriate test or clearly documented as unsupported.

Written by the indexing model from the issue text.