Add SigningService & SigningConfig during operation construction (#202)

* Add SigningService & SigningConfig during operation construction

This commit adds:
- SigV4SigningFeature during operation construction
- A `signing_service()` function to config objects to define a default signing service

These are both gated on the `aws.auth#sigV4` trait being present on the service

* Fix sig-auth missed changes

* Fix bad import