Cherry-pick #21406 to 7.x: Split index restrictions into type,dataset, namespace parts

[michalpristas]

Cherry-pick of PR #21406 to 7.x branch. Original message:

What does this PR do?

This PR does not apply same restrictions set on all parts of resulting index and follows 20/100/100 bytes length requirement for type, dataset and namespace

Why is it important?

Makes rules in par with the ones applied at fleet level

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.

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #21459 opened]

• Start Time: 2020-10-02T08:07:25.753+0000

• Duration: 35 min 30 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1384
Skipped	4
Total	1388

[ph]

Backport LGTM