svchost/auth: HostCredentialsFromMap function

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.

svchost/auth/from_map.go

@@ -0,0 +1,18 @@
+package auth
+
+// HostCredentialsFromMap converts a map of key-value pairs from a credentials
+// definition provided by the user (e.g. in a config file, or via a credentials
+// helper) into a HostCredentials object if possible, or returns nil if
+// no credentials could be extracted from the map.
+//
+// This function ignores map keys it is unfamiliar with, to allow for future
+// expansion of the credentials map format for new credential types.
+func HostCredentialsFromMap(m map[string]interface{}) HostCredentials {
+	if m == nil {
+		return nil
+	}
+	if token, ok := m["token"].(string); ok {
+		return HostCredentialsToken(token)
+	}
+	return nil
+}