Add support for "credential_process"

[laurrentt]

I'm using the credential_process config in .aws/credentials. For ex:

[my-1p-profile]
credential_process = sh -c "op get item 'AWS -...

This allows me to pull my access key and secret key pair from a password manager. But if I use this profile as the source_profile for assuming a role using assume-role, I get the following error:

panic: SharedConfigAssumeRoleError: failed to load assume role for arn:aws:iam::REDACTED:role/REDACTED, source profile has no shared credentials

It could be a great addition to support this type of configuration.