Cherry-pick #18685 to 7.x: Download snapshot artifacts from snapshots repo

[michalpristas]

Cherry-pick of PR #18685 to 7.x branch. Original message:

What does this PR do?

This PR introduces new downloader which tries to detect snapshot artifact repository in case agent is built as a snapshot.
If detection fails it just does not include snapshot downloader and proceed as usual (with disk backed up by configured downloader)

Why is it important?

More intuitive scenario when you build a snapshot without anything else and it downloads dependencies. Without this it would fail trying to download SNAPSHOT of 8.0.0 from official elastic.co

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.

cc @111andre111

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #19155 opened]

• Start Time: 2020-06-12T10:40:53.245+0000

• Duration: 32 min 11 sec

Steps errors

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• Name: Install Go 1.13.10

  • Description: .ci/scripts/install-go.sh

  • Duration: 1 min 22 sec

  • Start Time: 2020-06-12T11:03:43.391+0000

  • log

[blakerouse]

Backport looks good.