Cherry-pick #21736 to 7.x: Add missing scanner configuration of filestream

[kvch]

Cherry-pick of PR #21736 to 7.x branch. Original message:

What does this PR do?

This PR adds the missing configuration annotations to the scanner config.

Why is it important?

Now the following configuration can be accepted:

prospector.scanner.check_interval: 10s
prospector.scanner.exclude_files: ["something"]

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/integrations-services (Team:Services)

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Build stats

• Build Cause: [Pull request #21769 opened]

• Start Time: 2020-10-13T18:49:04.326+0000

• Duration: 70 min 29 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4447
Skipped	564
Total	5011

[michalpristas]

backport looks 🆗