Support credential_process for AWSClient

[madrob]

Describe the solution you'd like
Soto's AWSClient supports reading credentials from .aws/config and .aws/credentials files, but is missing support for the credential_process key - https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-sourcing-external.html

[madrob]

FYI, this was also implemented in the AWS SDK - awslabs/aws-sdk-swift#1076

[adam-fowler]

Sorry haven't responded to this. I was concerned there might be some security issues with this. I guess if AWS don't think there is then I can look into it