mme_registry: add backwards compat for old sctp_client config
When 'mme_pool' is absent from the config, automatically populate the MME pool with a single 'default' entry derived from the 'sctp_client' section (including legacy mme_loc_addr/mme_rem_addr params), and emit a deprecation warning. osmo_s1gw_sup:init/1 normalises 'sctp_client' in the app env (with all defaults applied) before children start, so mme_registry can safely read it without duplicating the parsing logic.
