This CredentialsSource can serve as an extension point to pass credentials
from an arbitrary external system to Terraform. For example, an external
helper program could fetch limited-time credentials from HashiCorp Vault
and return them, thus avoiding the need for any static configuration to
be maintained locally (except a Vault token!).
So far there are no real programs implementing this protocol, though this
commit includes a basic implementation that we use for unit tests.
Martin Atkins