HDDS-10481. Use proto 3 in SCMRatisProtocol.proto

[Russole]

What changes were proposed in this pull request?

• Add REQUEST_TYPE_UNSPECIFIED = 0 to align with proto3 enum default value semantics.
• Migrate SCMRatisProtocol.proto from proto2 to proto3 by replacing required fields with optional.
• Reserve field numbers and names to prevent accidental reuse in future schema changes.
• Add required-equivalent presence checks during deserialization, since proto3 no longer validates required fields when deserialization.
• Add unit tests to verify deserialization fails when required fields are missing

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-10481

How was this patch tested?

• All CI checks passed.
  • https://github.com/Russole/ozone/actions/runs/21537543811
• All added unit tests pass.

[szetszwo]

@Russole , thanks for working on this!

I suggest to move the old proto to test and rename the outer classname to something like Proto2SCMRatisProtocolForTesting. Then, we can create objects, serialize them using Proto2SCMRatisProtocolForTesting (proto2) and deserialize them using SCMRatisProtocol (proto3) for testing compatibility.