Support credential_source

[OrangeDog]

In order to use the built-in credentials of an AWS instance, this profile property needs to be supported.

credential_source - The credential provider to use to get credentials for the initial assume-role call. This parameter cannot be provided alongside source_profile. Valid values are:

Environment to pull source credentials from environment variables.
Ec2InstanceMetadata to use the EC2 instance role as source credentials.
EcsContainer to use the ECS container credentials as the source credentials.

[jdowning]

@OrangeDog Hi! This seems reasonable. Would you be able to propose a change with a PR?