Cherry-pick #21480 to 7.x: Add kubernetes composable inputs provider

[exekias]

Cherry-pick of PR #21480 to 7.x branch. Original message:

What does this PR do?

Add kubernetes dynamic provider for Pods, this will allow to launch input configurations for discovered Pods in a Kubernetes cluster. Example config:

providers.kubernetes:
  kube_config: /Users/exekias/.kube/config
  node: minikube

inputs:
  - type: logfile
    streams:
      - paths: /var/log/containers/*${kubernetes.container.id}.log

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

Closes #19271

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[blakerouse]

Backport looks good.

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #21586 opened]

• Start Time: 2020-10-06T16:38:38.431+0000

• Duration: 38 min 16 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1386
Skipped	4
Total	1390