Add AWS workload federation support

[weaversam8]

Fixes #185.

• Adds a new Goth.AWS module which handles constructing a signed AWS request
  • (added ExAws as a dependency for signing these headers)
• Extends Goth.Token to defer to Goth.AWS when it receives a credential_source with environment_id set to aws1
• Adds a new test to validate the above behaviour

[weaversam8]

My most recent commit also extends all workload federation so that scopes can be passed when requesting a token. Previously, only the default scope would be passed.

[sekiyama58]

Thanks for your work! If you could support IMDSv2 (with the config file generated by gcloud iam workload-identity-pools create-cred-config --aws --enable-imdsv2 option) that would be great.
It would also be nice if ex_aws dependency could be optional for those who don't use aws federation.