Low-level API should return an unexected Request

[omus]

The various low-level Service types we provide all generate Request instances and then execute them via submit_request. We may want to consider having the low-level services return the unexecuted Request instance to allow for:

• ...inspecting/debugging of a Request before executing it
• ...specifying the AWS HTTP backend per API call (currently is just set globally)
• ...an improved interface for presigned URLs. The AWS SDK for JavaScript v3 allows the creation of a command (i.e. Request) which can be signed and used later (see @aws-sdk/s3-request-presigner)