helper/schema: CoreConfigSchema method for Backend

This is just like the method of the same name on Resource, adapting the
helper/schema model of schema into Terraform Core's idea of schema.

helper/schema/core_schema.go

@@ -154,3 +154,9 @@ func (s *Schema) coreConfigSchemaType() cty.Type {
 func (r *Resource) CoreConfigSchema() *configschema.Block {
 	return schemaMap(r.Schema).CoreConfigSchema()
 }
+
+// CoreConfigSchema is a convenient shortcut for calling CoreConfigSchema
+// on the backends's schema.
+func (r *Backend) CoreConfigSchema() *configschema.Block {
+	return schemaMap(r.Schema).CoreConfigSchema()
+}