By adding this method you now only have to pass a `*disco.Disco` object around in order to do discovery and use any configured credentials for the discovered hosts.
Of course you can also still pass around both a `*disco.Disco` and a `auth.CredentialsSource` object if there is a need or a reason for that!
This uses an in-memory table of credentials keyed on hostname. This is
the simplest possible credentials source that can actually return
credentials, and is suitable for representing statically-configured
credentials from configuration.
For situations where no credentials are needed but where a working
CredentialsSource is still required, this variable provides a convenient
way to get a fully-functional-but-empty credentials source.
This credentials source wraps another and caches its results in memory
in a map that is keyed on the request hostname.
This should ease the common case of many services operating on the same
hostname by allowing an initial set of credentials to be re-used for
subsequent requests.
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.
This function deals with turning a map derived from some user input
(e.g. in a config file) into a HostCredentials object, if possible. This
will be used as a standard way to specify credentials so we have a place
to add new credentials types in future and have support for those across
all of our map-based CredentialsSources.
