API: Add support for /authenticate request

[rtamalin]

The headers of the /authenticate request must include the following items:

• client id
• SHA256 of the existing authentication token

The payload of the /authenticate request contains the following items:

• SHA256 of the client instance id
  • optional, only required if a client is requesting a new token because the existing one has expired or is no longer valid

[rtamalin]

This underlying work for this was completed as part of #38.

The authentication approach was simplified to just requesting an updated client credentials set that includes a freshly generated auth token. The authenticate request requires a payload containing the client's id, and hash digest of it's instance id. These will be used by the server to validate that the client is registered with the server, and that the provided hash digest matches that of the client instance it provided when the client registered.