Cherry-pick #21372 to 7.x: Upgrade Action: make source URI optional

[michalpristas]

Cherry-pick of PR #21372 to 7.x branch. Original message:

What does this PR do?

This PR makes source URI optional and requires it on snapshot upgrades. Otherwise if it is missing agent uses source URI from elastic-agent configuration.

Making it optional enables us to be prepared for cases when fleet will support snapshot upgrades without a change.

Why is it important?

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 #21440 opened]

• Start Time: 2020-10-01T06:56:26.771+0000

• Duration: 36 min 57 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1390
Skipped	4
Total	1394

[blakerouse]

Backport looks good.