Support delayed auth with extended authorization

[nickajacks1]

Support MOSQ_ERR_AUTH_DELAYED for extended auth.
Added mosquitto_complete_extended_auth, which optionally sends auth data. Sadly, there's a bit of code duplication between this and handle__auth. I wasn't sure if it was worth splitting out into a shared segment of code or not.
Added two client states, mosq_cs_delayed_ext_auth and mosq_cs_delayed_ext_reauth. This seemed to be cleaner than trying to use existing client states.
Updated the delayed_auth plugin example to support extended auth.

I commandeered contexts_by_id_delayed_auth a bit. My understanding is that previously, that table was disjoint with contexts_by_id, but that won't work well for reauthentication. Since the same hash handle can't be used for more than one table, I added a new handle.

Maintainers, feel free to modify/discard any portion of this changeset as necessary.

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Fixes #3357

[nickajacks1]

I will be available to resolve conflicts only after Nov 1 at the earliest

[nickajacks1]

I've resolved the conflicts, but I still need to go back and double check my work. I don't trust the me from 3 months ago.