[http_endpoint input] Add support for including headers and preserving original events

[P1llus]

What does this PR do?

This adds the possibility to give a list of headers that, if they exist in the incoming request, will be added as a separate object. Also adds the possibility to preserve the raw event as part of event.original.

Example output here:

Config example:

- type: http_endpoint
  enabled: true
  listen_address: 10.10.10.10
  listen_port: 8080
  preserve_original_event: true
  include_headers: ["TestHeader"]

  "json": {
    "test": "somevalue"
  },
  "event": {
    "original": "{\"test\":\"somevalue\"}"
  },
  "headers": {
    "TestHeader": "test"
  }

Why is it important?

Certain modules is hard to parse data without the extra details from the headers, github webhooks is one example of that, in which the data type is stored in the header

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/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #26279 updated

• Start Time: 2021-06-22T10:29:15.458+0000

• Duration: 101 min 44 sec

• Commit: fd4ef01

Test stats 🧪

Test	Results
Failed	0
Passed	7385
Skipped	1201
Total	8586

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Test stats 🧪

Test	Results
Failed	0
Passed	7385
Skipped	1201
Total	8586

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b httpendpoint_header_original upstream/httpendpoint_header_original
git merge upstream/master
git push upstream httpendpoint_header_original