Improve AWS SigV4 support.

[vikhyat]

PR Checklist:

• I have run npm test locally and all tests are passing.
• I have added/updated tests for any new behavior.
  
• If this is a significant change, an issue has already been created where the problem / solution was discussed: N/A
  

PR Description

This makes the following changes to the AWS SigV4 signing functionality:

1. Sign all request headers instead of just Content-Type.
2. Allow specifying a service name.

[chasingSublimity]

Any chance we could get this merged? If there's other work necessary, I'd be happy to pick it up.