libosmo-mgcp-client: extend the mgcp_client for MGW pooling

At the moment the MGCP Client only supports one MGW per application.
Depending on the requirements of the application one MGW might not offer
the performance needed. Lets add support for an MGCP Client pool that is
backward compatible to existing applications.

Change-Id: Icaaba0e470e916eefddfee750b83f5f65291a6b0
Related: SYS#5091
diff --git a/include/osmocom/mgcp_client/mgcp_client_endpoint_fsm.h b/include/osmocom/mgcp_client/mgcp_client_endpoint_fsm.h
index 78e3a41..f21f40e 100644
--- a/include/osmocom/mgcp_client/mgcp_client_endpoint_fsm.h
+++ b/include/osmocom/mgcp_client/mgcp_client_endpoint_fsm.h
@@ -47,6 +47,7 @@
 const char *osmo_mgcpc_ep_name(const struct osmo_mgcpc_ep *ep);
 const char *osmo_mgcpc_ep_ci_name(const struct osmo_mgcpc_ep_ci *ci);
 const char *osmo_mgcpc_ep_ci_id(const struct osmo_mgcpc_ep_ci *ci);
+struct mgcp_client *osmo_mgcpc_ep_client(const struct osmo_mgcpc_ep *ep);
 
 extern const struct value_string osmo_mgcp_verb_names[];
 static inline const char *osmo_mgcp_verb_name(enum mgcp_verb val)