client: expose body, add generic request methods and streaming

client: expose body, add generic request methods and streaming

The get/put/post/put_without_body/... methods now have a default
implementation forwarding to a generic `request` method as all our
implementations do the same already anyway.

Additionally, in order to allow easy access to a "streaming body", the
Body type is now exposed.

In the future, this crate may also require a wrapper to standardize
the handling of `application/json-seq` streams if we end up using
them, but for now, a simple way to expose the body is enough to get
going.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>