Use `protovalidate` instead of `protoc-gen-validate` for inbound validation

[tstirrat15]

protovalidate is the functional successor to protoc-gen-validate. It'd be good to get SpiceDB using it for ingress validation: https://github.com/bufbuild/protovalidate-go

And then we can start removing protoc-gen-validate from the generation step of authzed-go.