Allow client to specify expiration time

[slifty]

Task

Description

AWS lets you specify a timeout for when a signed request will no longer work.

We should allow this to be passed to the endpoint in order to support a wider variety of workflows (currently we just default to 3600 seconds aka an hour).

[slifty]

Open question: does the AWS timeout mean the upload has to start by that time, or end by that time.