Use ruma-client for sending requests / receiving responses

[jplatte]

It would be nice to push the lower-level parts of converting Rust request / response types to / from whatever the HTTP client library needs to Ruma, which also has an implementation of this that doesn't directly interact with all the Rust SDK's configuration bits and such. With this, there would be a cleaner line between the opinionated API parts in here, and the very basic Matrix protocol interactions in Ruma.

Blocked on ruma/ruma-client#5, and possibly other Ruma improvements.

[ShadowJonathan]

And, with ruma-client, it'd have the advantage of fetching / integrating / knowing a server's API version before it'll perform any requests, which would help along low-level serialization/API versioning requirements according to the spec. (unstable vs r0 vs v3 versioned apis)

[jplatte]

Actually, that stuff has long been integrated into the SDK (#531) 🙂