feat: implement ProtobufNative schema

[BewareMyPower]

fixes #294

[Copilot]

Pull request overview

Implements support for Pulsar’s PROTOBUF_NATIVE schema type in the Python client, enabling producers/consumers to use native Protobuf binary encoding with a schema definition compatible with the Java client’s ProtobufNativeSchema.

Changes:

• Add ProtobufNativeSchema implementation that builds a fileDescriptorSet-based schema definition and encodes/decodes using Protobuf binary.
• Export SchemaType.PROTOBUF_NATIVE from the _pulsar extension and expose the new schema from pulsar.schema.
• Add unit tests and CI install updates for protobuf-backed schema tests.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
pulsar/schema/schema_protobuf.py	Adds ProtobufNativeSchema implementation and schema definition construction from Protobuf descriptors.
pulsar/schema/__init__.py	Exposes ProtobufNativeSchema from the schema package.
src/enums.cc	Exports SchemaType.PROTOBUF_NATIVE into the Python bindings.
setup.py	Adds a protobuf extra and reuses it from the functions extra.
tests/schema_test.py	Adds brokerless unit tests for ProtobufNativeSchema and imports generated test messages.
tests/test_schema.proto	Defines test Protobuf messages used by the new unit tests.
tests/test_schema_pb2.py	Adds checked-in generated Protobuf Python code for the test messages.
.github/workflows/ci-pr-validation.yaml	Installs wheel with [avro,protobuf] extras for CI unit tests.

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