Remove HTTP client creation

[Jake-Shadle]

After #109 and #104 (and #50) I've decided to change the structure of Ctx to force users to pass in their own ureq::Agent that is configured how they want it to be, rather than adding more and more customization into the library itself. xwin was never intended as a library, but since people are using it that way, I don't think it's a good idea to have the HTTP client customization in the library, especially since it's reading environment variables which IMO is not something libraries should do in most cases. This is a bit unfortunate since it means callers have to have their own code if they want to customize, but it means this crate is simpler, and caller's are free to make whatever decisions they want (other than HTTP client, but that's something that can be done later).

[Jake-Shadle]

Pinging @Owen-CH-Leung in case you have any comments on this. IMO moving the custom cert code to cargo-xwin will be cleaner and (I believe) removes xwin as another step of indirection in the chain of maturin <-> cargo-xwin <-> xwin

[Owen-CH-Leung]

@messense FYI. Feel free to voice your comment here too

[MarijnS95]

I would wait a little bit but it might be useful to use the http crate for client-agnostic requests and responses. The ureq crate had some support for it but it was rather sloppy/inconvenient to use, but that is all cleared up in their main branch and should make it into a release. Now converting between the crates is just one or two lines as it is supposed to be.

[messense]

Looks good to me.