[Snapshot] Mark all new proto message fields as optional

We should do this before snapshot feature is merged to master branch.

Context:
https://github.com/apache/ozone/pull/3824#discussion_r1011376445
https://github.com/apache/ozone/pull/3985/files#r1041505724

Look for all the .proto file changes in snapshot feature branch:

https://github.com/apache/ozone/compare/master...HDDS-6517-Snapshot