Properly split remote proxy http https

[arthurpassos]

This PR does 3 things:

1. Fixes small remote resolver bug - more info below.
2. Removes DB::ProxyConfiguration::Protocol::ANY
3. Adds proxy docs

Proxy servers must be picked based on the request protocol (i.e, https request should grab the https_proxy or whatever https related configuration). This was being done correctly for the environment and list proxy resolvers, but not for the remote resolver.

This is how it was implemented:

• The environment resolver would query http_proxy or https_proxy environment variables based on the request protocol.
• The list resolver would round-robin URIs listed in <proxy><http OR https></proxy></> based on the request protocol.
• The remote resolver, on the other hand, would look into the proxy server protocol, not to what the user configured as a resolver for that protocol (i.e, proxy_scheme) - This is wrong, it's perfectly fine to have a HTTPS request routed through a HTTP proxy. The comparisong that should be made is the same that is being done for list and environment, that is: request protocol x user configuration for that protocol.

This PR addresses this issue by changing the syntax of remote resolvers. It now has to be encapsulated into <http> or <https> tags just like the list resolver.

<http> // new
     <resolver>
           // rest as usual
     </resolver>
</http>
<https> // new
     <resolver>
           // rest as usual
     </resolver>
</https>

Not sure I should classify this as bug or improvement.

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Select remote proxy resolver based on request protocol, add proxy feature docs and remove DB::ProxyConfiguration::Protocol::ANY.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

Information about CI checks: https://clickhouse.com/docs/en/development/continuous-integration/

[robot-ch-test-poll2]

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[arthurpassos]

@tavplubix Could you please take a look at this?

[arthurpassos]

@tavplubix kind ping

[Enmk]

time is too unspecific, please add units like seconds, minutes, hours, etc... into the config node name.

[arthurpassos]

The old configuration was already like this, changing it will break backwards compatibility. I can modify this for the new version only, but code will be a bit more complex and syntax will be even more different.

I suggest we keep it like this, what do you think?

[Enmk]

When will CH check the next proxy configuration? Would it try proxy lists if the resolver doesn't respond?

[arthurpassos]

It will check the next proxy configuration if it does not find a configuration for the requests protocol. For instance: ClickHouse is trying to send a HTTPS request and there is a remote resolver configuration section, but only for HTTP. In this case, it will check list and fallback to environment if there is no list for HTTPS.

[arthurpassos]

@tavplubix kind ping :)

[arthurpassos]

@tavplubix daily ping :)

[tavplubix]

The changelog entry is too long and detailed. Please add a short user-readable description for the changelog (one line, a few sentences)

[arthurpassos]

The changelog entry is too long and detailed. Please add a short user-readable description for the changelog (one line, a few sentences)

I have moved all text above changelog entry and added a small sentence, could you please check if it is ok now?

[tavplubix]

Stateless tests (release) - #55844
Stress test (tsan) - #55890

[arthurpassos]

Thanks @tavplubix!