Add support for user-defined operator aliases

[jachris]

This PR adds support for user-defined operator aliases that can be defined in the config. For example:

vast:
  operators:
    anonymize_urls: |
      replace net.url="xxx"
    aggregate_flows: |
       summarize 
         pkts_toserver=sum(flow.pkts_toserver),
         pkts_toclient=sum(flow.pkts_toclient),
         bytes_toserver=sum(flow.bytes_toserver),
         bytes_toclient=sum(flow.bytes_toclient),
         start=min(flow.start),
         end=max(flow.end)
       by
         timestamp,
         src_ip,
         dest_ip
       resolution
         10 mins 

This allows composing more complex pipelines, for instance from vast | anonymize_urls | aggregate_flows | write json to stdout. The PR only affects the new pipelines, which will eventually be used everywhere. The new pipeline parser also looks for definitions in vast.pipelines instead of vast.operators. However, this behavior is immediately considered deprecated and only available for compatibility.

[dominiklohmann]

I think this is a very small feature that is incredibly useful to have. We discussed in Slack that I'll write the documentation for this in an upcoming PR.