Cherry-pick #9392 to 6.x: Add geo fields to add_host_metadata processor.

[andrewvc]

Cherry-pick of PR #9392 to 6.x branch. Original message:

** EDIT ** I've left the original issue below the break, but after discussion we added geo fields to the add_host_metadata processor instead of a new one. Original is below

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This carries over from the discussion from #8620 .

This adds a new processor that lets users easily add geo fields associated with the host that created the event. You would use it like so:

processors:
  - add_host_geo:
      name: MN HQ
      location: "44.977753, -93.265015"

It's debate-able whether ECS should actually let you put these under perhaps agent.geo. That's something we should discuss here.

One other question here, should we just fold this functionality under add_host_metadata? I believe that probably makes more sense. We agreed in #8620 to make this a separate processor, but with the data nested under host, that makes less sense IMHO.

[ruflin]

LGTM. I restarted the travis Mac build as it got stuck somehow. If Travis goes green, I'm good to merge.