Cherry-pick #22575 to 7.x: Packetbeat agent interface and procs configuration

[andrewstucki]

Cherry-pick of PR #22575 to 7.x branch. Original message:

What does this PR do?

This PR acts as a stop-gap to allow for grouped streams to configure a packetbeat sniffer. As discussed in #22227 this currently does the (dumb) thing of just grabbing an interface configuration from any of the streams -- so if you have multiple streams specified, you should make sure they all have the same interface configuration. The next step will be to do some additional lifecycle management changes and spin off new packet sniffers based on configurations that differ, removing the coupling.

I put in a couple of tests to demonstrate the funky, but currently expected, behavior.

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.

Related issues

• Relates [Elastic Agent] Shared Packetbeat Sniffer Configuration #22227

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

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Build stats

• Build Cause: [Pull request #22601 opened]

• Start Time: 2020-11-16T19:13:50.589+0000

• Duration: 42 min 47 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1081
Skipped	10
Total	1091

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	1081
Skipped	10
Total	1091