Update `turbot aws` with a method that works with `credential_process`

[lorengordon]

The credential_process argument of an aws config profile allows sourcing credentials from an external process. It also natively supports refreshing those credentials. See also:

• https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-sourcing-external.html

Effectively, all the option needs to do, is send the credentials to stdout as a json blob of the form:

{
  "Version": 1,
  "AccessKeyId": "an AWS access key",
  "SecretAccessKey": "your AWS secret access key",
  "SessionToken": "the AWS session token for temporary credentials", 
  "Expiration": "ISO8601 timestamp when the credentials expire"
}