Avoid rebuilding sigstore/protobuf-specs every time

[vlsi]

Currently, sigstore-java rebuilds sigstore/protobuf-specs every time which seems wasteful, and it makes imports like import dev.sigstore.proto. invalid on the first project import.

We should use implementation("dev.sigstore:protobuf-specs:0.3.2") rather than protobuf("dev.sigstore:protobuf-specs:0.3.2")

sigstore/protobuf-specs#341 is needed to fix this

See also:

• chore: avoid generating protobuf(proto-google-common-protos) protobufs every time #734

[loosebazooka]

I actually explicitly didn't do this because of compatibility issues. I wanted to build the protos here