Support obtaining `protobuf` schemas from `schema registry` for `grpc` services

[jfallows]

Describe the desired outcome from the user's perspective
As a developer, I want to deploy updated protobuf schemas for grpc services and protobuf message validation without having to restart Zilla.

Acceptance criteria

• grpc binding can retrieve protobuf schemas from schema registry
• updated protobuf schema retrieved when schema registry cache max-age elapses

Additional context
None

Tasks

• Integrate catalog with grpc binding to obtain protobuf schemas
• Update grpc binding reference docs
• Update grpc binding configurations in zilla-examples

[dudo]

Would this add the magic byte to the message?

[jfallows]

Would this add the magic byte to the message?

@dudo The grpc binding currently produces raw protobuf messages towards the grpc-kafka binding, and will continue to maintain compatibility even when fetching the protobuf schema from schema-registry catalog in zilla.yaml.

However, specifying the type of messages on the topic in kafka cache_client binding using schema-registry catalog will prefix the magic byte, the schema id and message indexes onto the kafka topic, allowing a Kafka client with protobuf serdes and schema registry to fully deserialize the message.