Implement plugins, transforms

[CapacitorSet]

As discussed in chat.

This implements plugins (in the Go sense) for the following features:

• Services;
• Transforms;
• Channels.

Currently, plugins are searched for in the data directory, and are expected to be .so files exporting a certain symbol (Service for services, Transform for transforms, Channel for channels).

Transforms are a new component that generalizes the notion of filters. They are functions that take one event and return zero, one or more events (which do not necessarily include the original one) - conceptually, it's a flatmap. They are the groundwork for supporting Yara matching (addressed in another PR), Lua scripting, and possibly other extensions.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1b595a7). Click here to learn what that means.
The diff coverage is 3.44%.

@@            Coverage Diff            @@
##             master     #284   +/-   ##
=========================================
  Coverage          ?   41.75%           
=========================================
  Files             ?       77           
  Lines             ?     8780           
  Branches          ?        0           
=========================================
  Hits              ?     3666           
  Misses            ?     4689           
  Partials          ?      425

Impacted Files	Coverage Δ
services/dns-proxy.go	0% <ø> (ø)
services/counterstrike.go	0% <ø> (ø)
services/tftp.go	0% <ø> (ø)
services/memcached.go	0% <ø> (ø)
server/honeytrap.go	3.1% <0%> (ø)
services/services.go	8.82% <6.25%> (ø)

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