kafka_consumer: Support Protobuf messages with schema registry

[piochelepiotr]

What does this PR do?

Fixes support for Confluent's Protobuf wire format message indices in the kafka_consumer integration. The integration now correctly extracts and uses message indices to deserialize Protobuf messages, including support for multiple message types and nested types.

The Confluent Protobuf wire format is structured as:

[magic_byte][schema_id:4bytes][message_indices_length:varint][message_indices:varint...][protobuf_payload]

Motivation

Confluent's Protobuf wire format includes message indices after the schema ID to specify which message type from the .proto file should be used. Without this fix, deserialization fails when schemas contain multiple message types or when using non-zero indices.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

❌ Patch coverage is 91.56627% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.01%. Comparing base (a5342b2) to head (b4e89d0).
⚠️ Report is 20 commits behind head on master.

Additional details and impacted files

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Review from dkirov-dd is dismissed. Related teams and files:

• agent-integrations
  • kafka_consumer/changelog.d/22020.fixed

Review from Kyle-Neale is dismissed. Related teams and files:

• agent-integrations
  • kafka_consumer/changelog.d/22020.fixed