config/sys.config: group pfcp_peer params into a map
Following the same pattern as sctp_{client,server}, group the flat pfcp_loc_addr/pfcp_rem_addr environment variables into a pfcp_peer map. The old flat keys are still supported for backwards compat.
Changes: * osmo_s1gw_sup: add pfcp_cfg(), merging legacy flat keys with the new pfcp_peer map (new takes priority); store the resolved config back via set_env(pfcp_peer, ...) so all consumers see a single canonical map * pfcp_peer: change start_link/2 to start_link/1 taking a cfg() map; simplify init() using sctp_common:parse_addr/1; add cfg() type * rest_server: read pfcp laddr/raddr from the pfcp_peer map
pfcp_peer: make assoc_setup and heartbeat_req timeouts configurable
Add assoc_setup_timeout and heartbeat_req_timeout as optional fields in the pfcp_peer config map, with 2000 ms defaults. Store the full cfg() map in #peer_state{} and read values from it with maps:get/3 at the point of use.
