Cherry-pick #17549 to 7.x: [Heartbeat] Refactor TCP Monitor

[andrewvc]

Cherry-pick of PR #17549 to 7.x branch. Original message:

What does this PR do?

Refactors the TCP monitor to make the code easier to follow, more testable, and fixes #17123 where TLS server name was not correctly sent. This is important because the code had accrued a lot of cruft and become very hard to follow. There were many wrappers and intermediate variable names that often subtly changed names as they crossed various functions. When debugging #17123 I frequently found myself lost tracing the execution.

This new code should be simpler to understand for a few reasons:

1. Less code (almost a 2x reduction) and fewer, simpler, better organized functions
2. Less variable passing/renaming due to use of struct for key config variables
3. More consistent and descriptive variable names
4. Creation of the dialer as late as possible, to remove the confusing partial states, and clarity as to when which dialer layers are used.
5. Adds (frustratingly tricky) integration tests for Heartbeat TLS servername set incorrectly #17123 using mismatched TLS certs, and also against a real SOCKS5 proxy
6. Adds, for testing only, the ability to override the real network resolver for TCP checks which is invaluable in debugging TLS checks that depend on setting hostnames correctly. In the future if we decide to let users use a custom DNS resolver this will be nice.
7. Reorganized giant TCP test file into multiple files

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• 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.

How to test this PR locally

Run the sample config in #17123

[elasticmachine]

Pinging @elastic/uptime (:uptime)