Adds functionality to `schema_registry_decode` to emit defaults or not

[hendoxc]

What

This PR allows users to specify different behaviour for schema_registry_decode when decoding protobuf messages; allowing them to chose to Emit defaults.

Why

if a protobuf message like {"a": 0, "b": 1} is encoded to protobuf, when we decode with schema_registry_decode the resulting message is {"b": 1} and the keyval "a": 0 is lost.

This behaviour is controlled from protojson.MarshalerOpts with the field EmitDefaultValues, which by fault is false.

[CLAassistant]

All committers have signed the CLA.

[ntx-ben]

Currently facing this issue. Any updates on this?