License enforcement

[dfawley]

CNCF requires a license notice on every source file (ref):

Where reasonably possible, each source code and documentation file that is contributed to a CNCF project should contain a copyright notice and license notice.

In gRPC-Go, we enforce this with an automated check in a github action:

https://github.com/grpc/grpc-go/blob/bb4b6d5b987f45b76fc99ceb39231e5ec53f0880/scripts/vet.sh#L37-L40

We should implement something similar in the tonic repo.