Provide an api for xApps (and potentially other components in the future) to specify a handler for A1-policy messages (create/delete).

The xApp only needs to write a handler code that receives the policy instance payload in JSON. and register this handler (with the list of supported policy types) with the framework.

The framework calls this handler whenever it receives the A1 policy message (A1_POLICY_REQ 20010). The the xApp registers the handler, the framework will automatically send the policy query (A1_POLICY_QUERY 20012) message (that will be routed to A1 Mediator).

Finally, the api provides an operation for the xApp to acknowledge the A1-policy message and the acknowledgement operation generates the appropriate RMR message (A1_POLICY_RESP 20011).