simple start, wrap anchor registration into plugin system, use miniupnpd as an example.

use registerAnchor() to register new anchors for pf into the main firewall script, we've hooked head/tail options and the different anchor types.
anchorToText() will extract the registered anchors back for the specified types and place in the script (head/tail) in text format for pf.

our new codebase was already hooked using legacy_bindings.inc, no additional includes needed to construct \OPNsense\Firewall\Plugin().