Cherry-pick #20293 to 7.x: Add certificate TLS verification mode

[urso]

Cherry-pick of PR #20293 to 7.x branch. Original message:

What does this PR do?

Adds certificate TLS verification mode similar to Kibana and Elasticsearch, essentially skipping the hostname match.

Why is it important?

This is especially useful in k8s-like environments where users may have a certificate in use for their publicly accessible host name, but use a different host name for intra cluster communication. For environments like this in Beats today our only option is to disable TLS verification entirely, but this allows us to keep the majority of verifications enabled without disabling it entirely.

We had to implement essentially the same functionality in ECK for reference, the main difference is that beats also implements cert pinning so there's a little bit of extra logic to combine the two custom verifications: https://github.com/elastic/cloud-on-k8s/blob/master/pkg/utils/cryptutil/tls_verify.go#L18

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

• [ ]

How to test this PR locally

This is something I had difficulty with since it wasn't clear to me how to build a new beat and then use it. I imagine we will want to add additional tests, but I added unit test coverage to start. This is my first libbeat PR and could definitely use guidance here.

Related issues

Closes #8164

Use cases

Screenshots

Logs

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Pinging @elastic/integrations-services (Team:Services)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #21024 updated]

• Start Time: 2020-09-10T10:41:52.477+0000

• Duration: 73 min 4 sec

Test stats 🧪

Test	Results
Failed	0
Passed	20150
Skipped	1863
Total	22013