Cherry-pick #23076 to 7.x: [Processors] Add network direction processor

[andrewstucki]

Cherry-pick of PR #23076 to 7.x branch. Original message:

What does this PR do?

This adds a network direction processor that handles network classification based on "perimeter-based" ECS values (inbound, outbound, internal, external).

This will allow us to add beats-side processing and direction classification of events when all you have is an event with two IP addresses.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #23079 opened

• Start Time: 2020-12-10T19:10:09.510+0000

• Duration: 74 min 10 sec

Test stats 🧪

Test	Results
Failed	0
Passed	17362
Skipped	1404
Total	18766

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Test stats 🧪

Test	Results
Failed	0
Passed	17362
Skipped	1404
Total	18766