Add support for InputStreamProxy REST return type

[s-ludwig]

Allows to process large response bodies without loading everything into RAM.

[Geod24]

Any chance that we could recognize https://dlang.org/phobos/std_range_interfaces.html too ?
I've been wanting to return ranges from my interfaces since forever.

[s-ludwig]

@Geod24: That should be relatively easy with a wrapped StreamInputRange and also seems to me like a reasonable extension. I'd just leave that for a separate PR.

[Geod24]

Fairly straightforward, LGTM

[s-ludwig]

@Geod24: I've briefly started to implement InputRange support, but I'd defer that to the next release, because there are too many questions w.r.t. handling complex range element types (in the form of stream based serialization support). We should first discuss a possible approach as part of #2324.