AWS: add support for roles

[rsyring]

The current AWS support only does temporary session credentials. We should support getting role credentials.

[bchopson]

This setup supports roles:

[profile starfleet]
region = us-east-2
mfa_serial = arn:aws:iam::1234567890:mfa/1password-devmachine
credential_process = env-config-aws starfleet
envconfig_1pass = op://starfleet/Employee/aws-starfleet/

[profile vulcan]
region = us-east-2
role_arn = arn:aws:iam::0987654321:role/allies
source_profile = starfleet
envconfig_1pass = op://starfleet/Employee/aws-starfleet/

Edit: I don't think credential_process is needed/allowed if referencing a source profile

[rsyring]

@bchopson have you tested this? If it works, does that mean we just need to document it and no additional implementation is needed?

[bchopson]

I can confirm this works, in the context of setting up terraform infrastructure, and we just need to document.