Support `credential_process` in `~/.aws/config`?

[nathanl]

The AWS CLI allows configuring a credential_process - an external command to run which returns credentials in a specified format. Eg:

[profile dev]
credential_process = my_command -user me -role some_role

In our case, we use this to periodically get a new security token, as you added support for in https://github.com/ex-aws/ex_aws/pull/144/files

Would you like a PR to support automatically and periodically running the credential_process command?

[nathanl]

PR to support just getting the credential_process config value: ~https://github.com/ex-aws/ex_aws/issues/664~ whoops that was #665