Refactor api.client module to offer url from endpoint

In the api.client module:

• The API instance should be injected as a dependency in the Client instance (composition).

• The API class should not be public, so prefix it with an underscore.

• Why API.reverse_url() needs arguments already in conn_handler ?

  • As the endpoint can define the scheme for sure, we can withdraw it and get it from the ConnHandler class.
  • But for that, the ConnHandler class need to set only one scheme!

• Expose a method to easily get the url of an endpoint (refactor reverse_url of API in public Client class).

If we do that:

• Set only one scheme (with http, https, ws, wss) in ConnHandler class We can do that:
• Get the api url from the connhandler without giving any scheme.