[Filebeat][http_endpoint input] Adds support for custom auth header names and secret

[P1llus]

What does this PR do?

Certain webhooks offers functionality to add a custom header and defined secret to each request to identify the source. This PR adds the possibility to configure the header name and secret that needs to be confirmed for every request, similar but not the same as basic auth.

Why is it important?

Required for certain modules like Zoom #20414

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.

Author's Checklist

• Updated docs
• Updated nosetests

Added nosetest for wrong configuration option, ensuring that anything not matching header+secret is dropped and that a successful match allows the request with the correct responses.
Tested with:

> INTEGRATION_TESTS=1 BEAT_STRICT_PERMS=false nosetests -v -s tests/system/test_http_endpoint.py 
Test http_endpoint input with correct auth header and secret. ... ok
Test http_endpoint input with empty body. ... ok
Test http_endpoint input with GET request. ... ok
Test http_endpoint input with malformed body. ... ok
Test http_endpoint input with missing basic auth values. ... ok
Test http_endpoint input with HTTP events. ... ok
Test http_endpoint input with wrong auth header and secret. ... ok
Test http_endpoint input with wrong basic auth values. ... ok
Test http_endpoint input with wrong content header. ... ok

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Pinging @elastic/siem (Team:SIEM)

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💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #20435 updated]

• Start Time: 2020-08-10T11:06:23.876+0000

• Duration: 52 min 44 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2446
Skipped	397
Total	2843

[P1llus]

I added the changes you requested @andrewstucki . I did not hardcode a header value though, as currently this does not have a "secret.enable true/false". So the config validation checks expects both to either be nil or both set to a value.

[P1llus]

Jenkins test this