This introduces two new verbs to the credentials helper protocol to store
and forget credentials, and uses them to implement StoreForHost and
ForgetForHost.
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