allow using httpcache without depending on reqwest directly

[tamird]

reqwest types are part of the public API via Client::custom. this is pretty inconvenient because downstream users have to take a direct dependency on reqwest in order to use httpcache. It is additionally inconvenient that the version of reqwest used by the downstream user must match (or be compatible with) the version used in octorust (or other clients).

Possible solutions:

• provide a constructor that takes httpcache but not the client.
• re-export the necessary reqwest types.

[augustuswm]

That all makes sense to me. We are trying to sort out some of the dependency headaches with #95 . But it doesn't resolve the reqwest dependency.

I think the easiest thing here would be to add a with_http_cache method that will allow replacing the httpcache after a client has been constructed.

[tamird]

That would be easy, but mutability also stinks :)

Another constructor would be ideal. While you're at it, octorust's client::new returns Result but the implementation is in fact infallible.