feat: add https proxy server implementation

[bliuchak]

This PR represents simplification of #602. It contains changes related only to HTTPS proxy server support.

Scope:

• https proxy server + backward compatibility

Out Of Scope (should be fixed separately):

• TLS overhead. It's not included in connection statistic.
• HTTPS upstream proxy. By default they should work out of the box, however we're missing test coverage for them. Also TLS overhead is not included in connection statistic when HTTPS upstream proxy is used.
• IPv6 tests with HTTPS proxy. In general IPv6 with HTTPS proxy should work with no issue, however I can't proof it in tests due to limitation of got-scrapting.

Note: I might miss some edge cases for HTTPS server proxy, however I think that current coverage is good enough and covers core logic fully (80/20) and is backward compatible (HTTP proxy server tests pass).

[jirimoravcik]

Thanks for the PR, looks great, had a few comments

[tobice]

Honestly I have no idea how proxy-chain works and my general knowledge of proxies is still limited 😄 So I'm leaving the actual functionality review to Jirka and Ludvík.

From my PoV the application code change is small, well documented and easy to read. No concerns there.

The tests, especially the new ones, would benefit from splitting into files. Also one scenario caught my attention due to its nested structure. I'm not sure why that's needed.

Finally, I'd expand on the PR description. You can e.g. add there the nice diagram from the original PR.