net/chrony: Improve configuration options

[Matthew-Otto]

The current chrony plugin provides insufficient options for configuration.

This PR adds support for the following source options on a per-source basis:

• NTS
• prefer
• iburst
• min/max poll
• interleaving

It additionally adds support for pool-type sources, an NTP data diagnostics tab, and the local stratum feature as requested in #5115

[fichtner]

What happened to fallbackpeers? It looks like you may need a migration script for the model and I want some of the reordering out of the current diff because it makes it harder than necessary.

[Matthew-Otto]

Fallbackpeers is unnecessary now. Simply add a source without NTS marked. This is equivalent to how it works currently (authselectmode mix is the default behavior of chrony).

A migration script seems necessary. I have no idea how to implement that though. Can you point me to any resources on how that should work in opnsense?

Can you elaborate on what you mean by reordering? I don't follow.