[Filestream] New Input package

[P1llus]

What does this PR do?

This PR adds a custom input package for the filestream input, similar to how we have input packages for other similar custom inputs.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

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

[P1llus]

While the package is now in a working state, there is still some work that needs to be done on the documentation side, especially:

1. The README.md is currently just the same as from the filebeat documentation, do we want to update it or keep it?
2. The description for UI options in manifest.yml can be quite long, they are also based on the documentation we already have for those fields.
3. We need to make sure all the UI fields have the correct type (bool or text).

[P1llus]

Working with @kvch to see what we can do on the documentation front. Maybe we could minimize the documentation in the UI, and link more to the documentation page?

[elasticmachine]

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• Start Time: 2022-04-27T12:18:35.422+0000

• Duration: 13 min 46 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3
Skipped	0
Total	3

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (0/0)	💚
Files	100.0% (0/0)	💚 2.857
Classes	100.0% (0/0)	💚 2.857
Methods	100.0% (3/3)	💚 11.905
Lines	100.0% (0/0)	💚 11.167
Conditionals	100.0% (0/0)	💚