Allow multiple CDSServices

[jenniferjiangkells]

Add a service registry to allow registration of multiple CDSServices by id, and allow users to specify the id instance to send requests to in the client through the client decorator config.

The expected behaviour of a real CDS service is that there may exist multiple CDS services in a single service provider (which is listed through the CDS discovery endpoint). Currently, only 1 CDS service is allowed in the client, and the id is hard coded "1". The CDS discovery endpoint is also hardcoded with an example text.