Add ability to specify custom http client

[MrFoxPro]

I want to be able to connect to my s3 instance via unix domain socket, so it would be nice to have feature to configure or specify custom http client to achieve this.

[durch]

@MrFoxPro I was thinking about something like that a while ago, it would to be done in a similar fashion to the way async runtimes are implemented, ie a Trait, and then implementations for specific clients. I might come up with a trait and implementations for different clients, so folks could contribute implementations for different clients.