Users should be able to modify requests at a high level

[Velfi]

There is a common set of user-reported issues that could be solved by making high-level requests easier to modify.

This change would support these use cases and more:

• Sending unsigned requests to S3.
• Adding custom headers
• Augmenting the user agent sent with requests

Implementers adding a new method to an operation should take care to avoid namespace issues with the new method name(s).

A possible implementation would involve users passing a closer that took a request as input and then returned a request.

Related Issue(s):

• How to pass X-Amzn-Trace-Id with the SDK? awslabs/aws-sdk-rust#389

[jdisanti]

This was completed in #1647.