Switch Request.body to a Stream-like type

[chuwy]

Currently its String which means every time request is sent or received - we load the whole body into memory, which:

1. Might be unnecessary - in some cases we know we don't need the body based on request metadata
2. Blocking and takes up a lot of memory - if the request is 1Gb - it will be loaded into memory
3. Might not be a string, but a blob

There's IO.FS.Stream, but I'm not sure how good it fits.

[chuwy]

In worst case it can be IO ByteArray I think