SDK: Add library interfaces to support external workflows pre-registering telemetry clients

[rtamalin]

A registered telemetry client expects the following values to be configured:

• upstream telemetry server
• client instance id
• client credentials, consisting of the client id and the auth token
• optional customer id

If a telemetry client self-registers, it will automatically generate a client instance id, and use it to perform a client registration with the configured upstream server, defaulting to the primary SUSE Telemetry Gateway server, to obtain the client credentials.

If a telemetry client is being pre-registered using by an external product registration workflow, then these values will need to be configured appropriately by the associated tools, e.g. suseconnect.

To assist with integrating with external tools the telemetry library SDK should provide the following calls:

• GetUpstreamServer(), SetUpstreamServer()
• GetClientInstanceId(), SetClientInstanceId()
• GetClientCredentials(), SetClientCredentials()
• GetCustomerId(), SetCustomerId()