Handle pre-signed URLs

[grzm]

This feature isn't yet supported in aws-api. The goal of awyeah-api is to have feature parity with aws-api, so this won't be added until aws-api supports this feature.

The issue is currently being tracked in was-api here: cognitect-labs/aws-api#5

[grzm]

@jacobemcken has an implementation at https://github.com/jacobemcken/aws-simple-sign