make ipset type configurable

[jakobwenzel]

Currently, the type of ipset is hardcoded to be nethash. This type is not supported by the kernel in my Synology DS218+:

INFO[01-10-2022 20:58:58] ipset set-up : /usr/sbin/ipset -exist create crowdsec-blacklists nethash timeout 300
FATA[01-10-2022 20:58:58] iptables init failed: error while creating set : exit status 1 --> ipset v7.15: Kernel error received: set type not supported
FATA[01-10-2022 20:58:58] iptables init failed: error while creating set : exit status 1 --> ipset v7.15: Kernel error received: set type not supported

@zmeel seems to have run into the same issue: #32 (comment)

This PR introduces a configuration switch to allow the user to select an ipset type, while still defaulting to nethash. I'm not sure which other set type is optimal for my Synology, but with this PR applied, adding

ipset_type: hash:ip

to the config file makes the bouncer work.

[LaurenceJJones]

Hey thank you opening a PR! I will check this out soon.