[fix][schema]Fix AutoProduceBytes producer can not be created to a topic with ProtoBuf schema

[liangyepianzhou]

Motivation

1. There is a topic1 with a protobuf schema.
2. Create a producer1 with AutoProduceBytes schema.
3. The producer1 will be created failed because the way to get the schema of protobuf schema is not supported. ### ###

Modification

Because the Protobuf schema is implemented from the AvroBaseStructSchema. So we add a way to get Protobuf schema just like the AvroSchema.

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository:

[lhotari]

I wonder how this change makes sense. The condition is about SchemaType.PROTOBUF and then the schema is set to be GenericAvroSchema. This doesn't look right.