Add support for constructing custom proxy routing.

[jeeger]

• Modified ->ProxySelector to take a vector of handlers and proxy parameters in addition to the "simple" host/port config argument.
• Added functions to construct handler/proxy pairs.
• Added some tests for proxy selection
• Updated changelog.

Please answer the following questions and leave the below in as part of your PR.

• I have read the developer documentation.

• This PR corresponds to an issue with a clear problem statement.

• This PR contains a test to prevent against future regressions

• I have updated the CHANGELOG.md file with a description of the addressed issue.

[borkdude]

My initial reaction: why would we pass a vector of functions? I think we can cover all use cases with just one function right?

[jeeger]

I'll give it a go and see how the one-function interface would look.

[borkdude]

What I mean is, one could write all the conditions basically as one function with a cond instead of providing multiple separate functions, right?

[borkdude]

Feebdack:

It seems ->ProxySelector now lost the ProxySelector passthrough (the case where you pass something that is already a ProxySelector). Perhaps lock this down in a test as well?

The default client has a behavior change now, it will suddenly respect env vars. I think this should be opt-in since this is breaking. Since java.net.http doesn't respect env vars by default, I think we should just expose a function for this.

Maybe we could do this as a second PR as to not make this PR contain too many features at once.

[jeeger]

Good points, I'll split the "default proxy" behavior into its own pull request and add the "passthrough" case.

[jeeger]

This is done now.

[jeeger]

It helps to actually run the tests, fixed the test error.