update config handling to support internal/external split and adjust types

Signed-off-by: aramissennyeydd <aramis.sennyey@doordash.com>

packages/backend-plugin-api/src/alpha/index.ts

@@ -40,5 +40,4 @@ export const systemMetadataServiceRef = createServiceRef<
 export type {
   BackstageInstance,
   SystemMetadataService,
-  Target,
 } from './services/definitions/SystemMetadataService';

packages/backend-plugin-api/src/services/definitions/SystemMetadataService.ts

@@ -14,14 +14,12 @@
  * limitations under the License.
  */
 
-/** @alpha */
-export type Target = string | { internal: string; external: string };
-
 /**
  * @alpha
  */
 export interface BackstageInstance {
-  url: Target;
+  internalUrl: string;
+  externalUrl: string;
 }
 
 /**