Implement the non-streaming admin commands into frontend/grpc...

Problem to solve

Implement the non-streaming admin commands in frontend/grpc.

Stakeholders

The CTA team.

Proposal

- [ ] Create a gRPC version of cta-admin, with associated configuration and Kerberos authentication code. - [ ] Add in the non-streaming admin commands (not currently implemented) to use the AdminCmd class

• Add implementation of non-streaming commands that uses the AdminCmd class, without authentication
• Add support for authentication with gRPC tokens
• Add support for authentication with Kerberos