Document support for http over unix sockets and named pipes in Metricbeat

[jsoriano]

What does this PR do?

• Add mention in common HTTP docs about supporting http+unix and http+npipe schemes to connect to UNIX sockets and Windows named pipes.
• Add missing references to the common HTTP docs in modules that use the default URL parser and HTTP helpers.
• Add test for http+unix using the default URL parser and the HTTP helper together.

Why is it important?

• To better document support for HTTP over UNIX sockets.

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.

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Pinging @elastic/integrations (Team:Integrations)

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💚 Build Succeeded

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Build stats

• Build Cause: Pull request #22891 opened

• Start Time: 2020-12-03T13:04:29.195+0000

• Duration: 62 min 26 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2848
Skipped	630
Total	3478

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	2848
Skipped	630
Total	3478

[ruflin]

great to see this reusable component. @mtojek @ycombinator Would be great to have something like this also for the build process of integrations to pull in shared doc snippets.