gRPC Java S2A protos

[rmehta19]

Context:

• s2a: Add gRPC S2A grpc-java#11113
• https://github.com/google/s2a-go/blob/main/README.md

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: rmehta19 / name: Riya Mehta (e1d5e30, 195d173, c6b5677)

[matthewstevenson88]

Thanks for the PR! This change LGTM. Please fix the CLA issue, remove the "draft" piece from the title, and in the PR description can we link to https://github.com/google/s2a-go/blob/main/README.md ?

[rmehta19]

@matthewstevenson88 Thanks for the review! Done.

[matthewstevenson88]

LGTM.

Please get approval from @ejona86 as well. (I think I technically have the permission to approve this, but I'm not one of the normal maintainers of this repo so I'd like to get Eric's sign-off that he is OK with where we put the protos.)