Cherry-pick #21285 to 7.x: Send snapshot flag together with metadata

[michalpristas]

Cherry-pick of PR #21285 to 7.x branch. Original message:

What does this PR do?

For correct Source URI resolution fleet needs to be aware whether or not agent is a SNAPSHOT build or not.
This PR includes this information in two forms in local metadata which is sent during enroll and checkin

• elastic.agent.snapshot - additional field set to true if agent is in snapshot mode
• elastic.agent.build.original - long version info, in case of snapshot it will contain it e.g.8.0.0-SNAPSHOT (build: 3f1e3a5d016487f4cc995b019a846f3f13104f03 at 2020-09-24 08:57:34 +0000 UTC)

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 #21460 opened]

• Start Time: 2020-10-02T08:09:54.541+0000

• Duration: 35 min 28 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1390
Skipped	4
Total	1394

[blakerouse]

Backport looks good.