Add LookupHost expr lib func

[ThinkChaos]

Makes it possible to allow-list a domain whose IPs are not known in advance.
A practical is example is whitelisting a home IP with a dynamic DNS setup without having to resort to allowing the whole country or ISP.

Example:

name: me/my_cool_whitelist
description: lets whitelist our own IP
whitelist:
  reason: dont ban my IP
  expression:
    - evt.Overflow.Alert.Source.IP in LookupHost('my-home-domain.invalid')

This is my solution to #1315 and crowdsecurity/hub#405. I'll let you decide if it fixes those requests, but my use case is the exact same so I think it should suffice.
I didn't implement it via a enrichment because it's easier this way: for this use case the param to the function is constant, so it doesn't need to be via enrich. Also enrich functions can only return a string (and no error).

[codecov-commenter]

Codecov Report

Merging #1775 (edc03a1) into master (6798dd7) will increase coverage by 6.60%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1775      +/-   ##
==========================================
+ Coverage   42.30%   48.90%   +6.60%     
==========================================
  Files          33      118      +85     
  Lines        6839    17737   +10898     
==========================================
+ Hits         2893     8674    +5781     
- Misses       3556     8044    +4488     
- Partials      390     1019     +629     

Flag	Coverage Δ
func-crowdsec	45.84% <ø> (ø)
func-cscli	41.68% <ø> (ø)
unit-windows	53.04% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/exprhelpers/exprlib.go	71.90% <100.00%> (ø)
pkg/acquisition/modules/kinesis/kinesis.go	0.00% <0.00%> (ø)
pkg/csconfig/profiles.go	73.07% <0.00%> (ø)
pkg/parser/enrich_geoip.go	43.00% <0.00%> (ø)
pkg/leakybucket/overflow_filter.go	75.75% <0.00%> (ø)
pkg/apiserver/utils.go	0.00% <0.00%> (ø)
...on/modules/syslog/internal/parser/rfc5424/parse.go	69.53% <0.00%> (ø)
pkg/parser/node.go	70.87% <0.00%> (ø)
pkg/cwhub/download.go	52.17% <0.00%> (ø)
pkg/apiclient/decisions_service.go	42.50% <0.00%> (ø)
... and 76 more

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[buixor]

Hello,

I like it, I'll keep you posted soon.

[LaurenceJJones]

Will add some documentation to docs.crowdsec.net to advise this should only be used in postoverflows as its very slow.