feat(caps): add cache clearing to /caps

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

src/xmpp/capabilities.h

@@ -28,5 +28,6 @@ void caps_map_jid_to_ver(const char* const jid, const char* const ver);
 gboolean caps_cache_contains(const char* const ver);
 GList* caps_get_features(void);
 char* caps_get_my_sha1(xmpp_ctx_t* const ctx);
+void caps_cache_clear(void);
 
 #endif