feat(caps): add cache clearing to /caps

Add /caps clear to wipe the local capabilities cache.
Implement autocomplete for new subcommand.

tests/unittests/xmpp/stub_xmpp.c

@@ -500,6 +500,11 @@ caps_jid_has_feature(const char* const jid, const char* const feature)
     return FALSE;
 }
 
+void
+caps_cache_clear(void)
+{
+}
+
 gboolean
 bookmark_add(const char* jid, const char* nick, const char* password, const char* autojoin_str, const char* name)
 {