Make message decoding in ProtobufDecoder more extensible

[AbhishekLaddha54]

Fixes gh-37265

Summary

ProtobufDecoder recently gained extensibility for size reading via MessageSizeReader (7.0), but message creation is still hard-coded in a private inner class. This makes gRPC-Web support impossible without copy-paste.

gRPC-Web uses:

• 5-byte header: 1 byte flag (0x80 = trailer, 0x01 = compressed) + 4 bytes BE length
• Trailer frames that must be skipped

Changes

• Add protected ExtensionRegistry getExtensionRegistry()
• Add protected @Nullable Message decodeMessage(CodedInputStream, ResolvableType) and overload for Class<?> — returning null skips frame
• Refactor decode(DataBuffer, ...) and MessageDecoderFunction to use decodeMessage()
• Make MessageDecoderFunction protected with getters and add createMessageDecoderFunction() factory

This mirrors the existing initMessageSizeReader() pattern.

Enables (example)

class GrpcWebDecoder extends ProtobufDecoder {
  @Override protected MessageSizeReader initMessageSizeReader() {
    return new GrpcWebReader(); // reads 5-byte header
  }
  @Override protected Message decodeMessage(CodedInputStream s, ResolvableType t) throws Exception {
    if (isTrailer) return null;
    return super.decodeMessage(s, t);
  }
}

See spring-projects/spring-grpc#427

Testing

./gradlew :spring-web:test --tests "ProtobufDecoderTests" → BUILD SUCCESSFUL, 0 failures

Existing tests unchanged, new hook returns same as old logic by default
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