CODEOWNERS: only apply to top-level API protos (#1553)
CODEOWNERS: only apply to top-level API protos (#1553)

Commit d0c1b7b7f05e ("Create CODEOWNERS file (#1520)") added
ownership to .proto files, but it applies to all .proto files
across the repo.

Instead, just apply to the top-level API definitions and storage definitions.

google/trillian,gdbelvin/trillian,Martin2112/trillian,daviddrysdale/trillian,google/trillian,daviddrysdale/trillian,Martin2112/trillian,AlCutter/trillian,RJPercival/trillian,RJPercival/trillian,gdbelvin/trillian,daviddrysdale/trillian,AlCutter/trillian,gdbelvin/trillian,Martin2112/trillian