Initial discussion template for gnoi.authzn.

[robshakir]

 * (A) auth/authzn.proto
  - Initial proposal for a local authorization and authentication
    service for network elements.

@aashaikh @danielywong @johnwestbrook -- PTAL.

[robshakir]

From our meeting on 7th January 2021:

• We will split out the RPCs so that we can have per-service authorisation on an RPC-level.
• Must users already be created before they can have their access control changed?
  • We consider that adduser or another user management tool (e.g., gNMI OpenConfig system/aaa model) would give us the way to be able to add new users.
• Is the complete set to be sent, or should partial configurations be permissible?
• We need to specify both the authorised users (certificates), and authorised keys (public keys). We need to be able to specify both with each system user (account).