plugin XEP-0033: code modernisation, improve delivery, data validation: - Code has been rewritten using Pydantic models and `async` coroutines for data validation and cleaner element parsing/generation. - Delivery has been completely rewritten. It now works even if server doesn't support multicast, and send to local multicast service first. Delivering to local multicast service first is due to bad support of XEP-0033 in server (notably Prosody which has an incomplete implementation), and the current impossibility to detect if a sub-domain service handles fully multicast or only for local domains. This is a workaround to have a good balance between backward compatilibity and use of bandwith, and to make it work with the incoming email gateway implementation (the gateway will only deliver to entities of its own domain). - disco feature checking now uses `async` corountines. `host` implementation still use Deferred return values for compatibility with legacy code. rel 450