Add schema registry support to ProtoBufMessageDecoder

[xiangfu0]

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.