Network direction processor additions

[andrewstucki]

This adds some functionality to the new network_direction processor that allows the processor to use templates for specifying internal_networks or to read the values as an array from a given field. This is important because it allows for dynamic processor execution based on the contents of a given field. We actually use this internally in a number of beats modules where we pass configuration from a configuration file up to a pipeline on the document being ingested--the processor then gets executed based off of that extra configuration context prior to the fields being dropped.

One of my first desired use cases is in the cisco umbrella filebeat module where I'm planning to add the following to the pipeline:

  - network_direction:
      internal_networks_field: _conf.internal_networks
      ignore_missing: true

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[danhermann]

@andrewstucki, can you update this PR with the new location of these files to resolve the merge conflicts?

[andrewstucki]

@danhermann sorry about the delay, this should be up-to-date now.

[danhermann]

Thanks, @andrewstucki. This looks pretty good although I think there are some simplifications that can be made as noted below.

[danhermann]

@andrewstucki, two small requests here and then we can merge this. Can you add a test in NetworkDirectionProcessorFactoryTests that validates the internal_networks_field option for the processor. And then one other small request below.

[danhermann]

Thanks, @andrewstucki. I'll get this merged and backported.

[danhermann]

cc: @elastic/es-ui in case auto-complete needs to be updated to accommodate this new option.