Add event counter

[CapacitorSet]

Fixes #303, depends on #284.

Adds an event counter with a configurable period.

Example configuration:

[[filter]]
channel=["console"]
groupby=["category"]
period=5

This configuration groups events by the "category" key, so that eg. if one receives 10 HTTP requests in 3 seconds, the following event will be emitted:

transforms > group > original-category=http, group.count=10, token=[...], date=[...], category=group, sensor=transforms

Note that the event is fired 5 seconds after the first event, so a series of requests across 12 seconds will result in 3 separate events.

This feature opens interesting scenarios if used together with plugins (creating filters that could implement "count the number of root logins per hour", for example), which should probably be documented together with this feature. However, it's also somewhat pushing the limits of the "expressive power" granted by Honeytrap, which only allows for a data flow with a fixed order for transforms (eg. you can't use a filter after a plugin) and a fixed amount of transforms (there can only be one plugin, one group-by), an issue which might be worth looking into.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@674ac4f). Click here to learn what that means.
The diff coverage is 2.5%.

@@            Coverage Diff            @@
##             master     #312   +/-   ##
=========================================
  Coverage          ?   43.57%           
=========================================
  Files             ?       70           
  Lines             ?     8336           
  Branches          ?        0           
=========================================
  Hits              ?     3632           
  Misses            ?     4288           
  Partials          ?      416

Impacted Files	Coverage Δ
services/memcached.go	0% <ø> (ø)
services/dns-proxy.go	0% <ø> (ø)
services/counterstrike.go	0% <ø> (ø)
services/tftp.go	0% <ø> (ø)
server/honeytrap.go	3.05% <0%> (ø)
services/services.go	8.82% <6.25%> (ø)

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