Support for connect-and-resume conduits

[endgame]

In amazonka-s3-streaming, @axman6 accepts a stream of data to send to S3 as a multipart upload. To send each upload, the library must first draw from the stream to create a request body.

In conduit, connect-and-resume streams are their own separate type, which means we probably want to think about handling them separately - perhaps by taking in a SealedConduitT and a maximum number of bytes to read?

[endgame]

This is probably not necessary: we would just be doing the same sealing/unsealing behaviour that anyone else could do with conduit themselves. That should be a separate library in the conduitverse.