net/haproxy: add SNI support in mapfile-based backend selection

[cluck]

This adds two options to the "Map domains to backend pools using a map file". One of the options is addressing https://github.com/opnsense/plugins/issues/3641.

The first option adds all map_dom, map_str, map_beg, map_end and map_reg (regex) support.

The second option adds SNI support.

Defaults are chosen such that existing setups don't change behavior.

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I am currently testing this in our lab, but the more testing this gets the better.