Optimize integration testing

- Add injection package plug-in
- Add libstellar_dynamic.so to facilitate unit testing of upper-level plug-ins

test/packet_inject/stellar_plugin/spec.toml

@@ -0,0 +1,4 @@
+[[plugin]]
+path = "./libpacket_inject_plugin.so"
+init = "packet_inject_plugin_init"
+exit = "packet_inject_plugin_exit"