add interface for getting configurations (#295)

* add interface for getting configurations

Signed-off-by: nirvanagit <anubhavaeron@gmail.com>

* Update registry.go

---------

Signed-off-by: nirvanagit <anubhavaeron@gmail.com>

admiral/pkg/registry/registry.go

@@ -0,0 +1,10 @@
+package registry
+
+// IdentityConfiguration is an interface to fetch configuration from a registry
+// backend. The backend can provide an API to give configurations per identity,
+// or if given a cluster name, it will provide the configurations for all
+// the identities present in that cluster.
+type IdentityConfiguration interface {
+	GetByIdentityByName(identityAlias string) error
+	GetByClusterName(clusterName string) error
+}