gbproxy: Move the VTY code into the vty file and create public API

Create public accessors to the core of the peer to allow to
simplify the test and separate concerns.

Done with Jacob.
diff --git a/openbsc/tests/gbproxy/gbproxy_test.c b/openbsc/tests/gbproxy/gbproxy_test.c
index d5675d8..c3680f5 100644
--- a/openbsc/tests/gbproxy/gbproxy_test.c
+++ b/openbsc/tests/gbproxy/gbproxy_test.c
@@ -1241,6 +1241,7 @@
 	setlinebuf(stdout);
 
 	printf("===== GbProxy test START\n");
+	gbproxy_init_config(&gbcfg);
 	test_tlv_shift_functions();
 	test_gbproxy();
 	test_gbproxy_ident_changes();