Support self-signed certificate on outputs

[belimawr]

What does this PR do?

Adds ssl.ca_trusted_fingerprint option, if set to a HEX fingerprint of a root CA certificate, this certificate is added to
the trusted CAs (as if it was defined on ssl.certificate_authorities), then the SSL validation continues as normal.

This happens during the SSL handshake.

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.

Author's Checklist

• Make sure the UX is correct

How to test this PR locally

1. Run an Elasticsearch 8.0 Instance with self signed certs
2. Edit the Elasticsearch output configuration from your beat:

output.elasticsearch:
  # Array of hosts to connect to.
  hosts: ["https://<your IP address>:9200"]

  username: "elastic"
  password: "<your elastic use password>"
  ssl.ca_trusted_fingerprint: "<ES CA certificate fingerprint>"

3. Run your Beat

Related issues

[mergify]

This pull request does not have a backport label. Could you fix it @belimawr? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2021-12-06T16:53:37.967+0000

• Duration: 134 min 1 sec

• Commit: ddc700a

Test stats 🧪

Test	Results
Failed	0
Passed	48589
Skipped	4276
Total	52865

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[kvch]

Please add the new option to the documentation, too: https://github.com/elastic/beats/tree/master/libbeat/docs/shared-ssl-config.asciidoc

[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 ca_sha256-option upstream/ca_sha256-option
git merge upstream/master
git push upstream ca_sha256-option

[belimawr]

rebased onto master, force push.