Changes

enb_proxy: split conn_info() into mme_conn_info() and proxy_info()

The old conn_info() conflated two distinct concerns: the MME SCTP
connection info stored in enb_registry (aid, saddr, sport) and the
broader operational state used for introspection (handler pid, enb
connection info, etc.).  Mixing them forced enb_registry to hold a
handler pid it has no business knowing about, and required rest_server
to extract that pid just to reach s1ap_proxy for E-RAB listing.

Split into two distinct types:

* mme_conn_info() - pure MME SCTP connection info (aid, saddr, sport),
  stored in the enb_registry and signalled via notify_mme_comm_up/2.
  The `mme_` prefix is dropped from field names as the type name
  provides the context.

* proxy_info() - richer operational snapshot (handler, enb_handle,
  enb_conn_info, mme_conn_info, genb_id_str, mme_info), returned by
  fetch_info/1 for introspection/debugging purposes.

Additionally:

* Add fetch_erab_list/1 to enb_proxy, delegating internally to
  s1ap_proxy:fetch_erab_list/1 via the cached handler pid.  This
  allows the rest_server to obtain a list of E-RAB without having
  to obtain pid of the s1ap_proxy and interact with it.

* Remove separate enb_aid/mme_aid/mme_saddr/mme_sport state fields;
  enb_aid is now read directly from enb_conn_info, and the MME fields
  are grouped in mme_conn_info.

Change-Id: Ia428ceb4762f972211e9b790688dc89fb5b8a274
Related: SYS#7066