Add schema registry support to ProtoBufMessageDecoder

### Motivation
`pinot-plugins/pinot-input-format/pinot-protobuf/src/main/java/org/apache/pinot/plugin/inputformat/protobuf/ProtoBufMessageDecoder.java` still carries a top-level TODO for schema registry support.

Pinot already has the Confluent-specific protobuf ingestion module, but the base protobuf decoder still requires descriptor files and class names. Supporting schema-registry-backed descriptor resolution here would close a long-standing gap, or clarify why the base decoder should stay file-based only.

### Scope
- Decide whether the base decoder should support schema registry directly or explicitly defer to the confluent plugin.
- If direct support is desired, define config and API behavior and add tests.
- Otherwise, replace the TODO with clear documentation pointing users to the supported path.

### Notes
Observed on `upstream/master` on April 15, 2026.