SDK: Add telemetry library support for the /authenticate API request

[rtamalin]

Leverage the /authenticate API request (#3) to implement authentication support in the telemetry library.

A client will use the /authenticate API request for one of the following reasons:

• to check that the existing auth token is still valid.
  • e.g. AuthenticationStatus()
  • potentially can also be used to check if telemetry client has been registered?
• to obtain an updated auth token as the existing one has expired
  • e.g. AuthenticationExtend()
• to obtain an updated auth token because the existing one is no longer valid
  • e.g. AuthenticationRenew()

The headers of the re-authentication request must include the following items:

• client id
• SHA256 of the existing authentication token

The payload of the re-authentication 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  • optional, only required if a client is requesting a new token because the existing one has expired or is no longer valid

The telemetry client will be responsible for storing the updated authentication token so that it can be used in future requests.

[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.

[rtamalin]

The auth tokens supplied by the server are now JWT's and it should be possible to check locally to determine if they have expired or now.

The server will fail with a HTTP 401 Unauthorized if an auth token is no longer valid, either because it has expired, or because the server had invalidated it, and it will be up to the client to obtain a new auth token using the authenticate request.