plugin: notify only the plugins with init state

This is fixing a bug that @chrisguida reported,
when we notify a plugin we also notify a plugin
that it is in the middle of initialization.

So imagine the following use case:

- 1. Plugin A sends get manifest
- 2. cln makes a check and generates a warning
- 3. Plugin A is subscribed to the warning or * notification
- 4. Core Lightning gets upset because it receives a warning message but it was waiting for a init.

I think it is still a bug in how Core Lightning handles the init, but also I think we should communicate with the plugin only if it is initialized.

```
lightningd-1 2024-04-23T23:20:34.154Z DEBUG   plugin-clnrest: Notification: {'warning': {'level': 'warn', 'time': '1713914433.818230531', 'timestamp': '2024-04-23T23:20:33.818Z', 'source': 'plugin-bookkeeper', 'log': \"topic 'utxo_deposit' is not a known notification topic\"}}
lightningd-1 2024-04-23T23:20:34.154Z DEBUG   hsmd: new_client: 0
lightningd-1 2024-04-23T23:20:34.154Z **BROKEN** plugin-test_libplugin: Did not receive 'init' yet, but got 'warning' instead
lightningd-1 2024-04-23T23:20:34.155Z INFO    plugin-test_libplugin: Killing plugin: exited before we sent init
lightningd-1 2024-04-23T23:20:34.155Z **BROKEN** plugin-test_libplugin: Plugin marked as important, shutting down lightningd!
lightningd-1 2024-04-23T23:20:34.155Z DEBUG   lightningd: io_break: lightningd_exit
lightningd-1 2024-04-23T23:20:34.155Z DEBUG   lightningd: io_loop: connectd_init
Time-out: can't find [re.compile('Server started with public key')] in logs
{'github_repository': 'ElementsProject/lightning', 'github_sha': '014c1eb383b0a65394cf166b3aa0174cf2077896', 'github_ref': 'refs/pull/7258/merge', 'github_ref_name': 'HEAD', 'github_run_id': 8808124001, 'github_head_ref': 'cguida/onchain_notif', 'github_run_number': 9395, 'github_base_ref': 'master', 'github_run_attempt': '1', 'testname': 'test_self_disable', 'start_time': 1713914432, 'end_time': 1713914612, 'outcome': 'fail'}
----------------------------- Captured stderr call -----------------------------
Did not receive 'init' yet, but got 'warning' insteadlightningd: lightningd/connect_control.c:767: connectd_init: Assertion `ret == ld->connectd' failed.
lightningd: FATAL SIGNAL 6 (version 014c1eb-modded)
0x555b1dc2d6f3 send_backtrace
	common/daemon.c:33
0x555b1dc2d8de crashdump
	common/daemon.c:75
0x7fda4584251f ???
	???:0
0x7fda458969fc ???
	???:0
0x7fda45842475 ???
	???:0
0x7fda458287f2 ???
	???:0
0x7fda4582871a ???
	???:0
0x7fda45839e95 ???
	???:0
0x555b1db98f8e connectd_init
	lightningd/connect_control.c:767
0x555b1dbb0307 main
	lightningd/lightningd.c:1249
0x7fda45829d8f ???
	???:0
0x7fda45829e3f ???
	???:0
0x555b1db6fd64 ???
	???:0
0xffffffffffffffff ???
	???:0
```

Reported-by: @chrisguida
Co-Developed-by: @chrisguida
Signed-off-by: Vincenzo Palazzo <[email protected]>

lightningd/plugin.c

@@ -2481,7 +2481,7 @@ bool plugin_single_notify(struct plugin *p,
 			  const struct jsonrpc_notification *n TAKES)
 {
 	bool interested;
-	if (plugin_subscriptions_contains(p, n->method)) {
+	if (p->plugin_state == INIT_COMPLETE && plugin_subscriptions_contains(p, n->method)) {
 		plugin_send(p, json_stream_dup(p, n->stream, p->log));
 		interested = true;
 	} else