Ensure that additional configs passed to loadBackendConfig are properly taken into account

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

.changeset/sour-cooks-repeat.md

@@ -0,0 +1,7 @@
+---
+'@backstage/backend-common': patch
+---
+
+Renamed the `loadBackendConfig` option `additionalConfig` to `additionalConfigs` as an array, and ensured that they get passed on properly.
+
+This is technically breaking, but the [original addition](https://github.com/backstage/backstage/pull/16643) hasn't been released in mainline yet so we are taking this step now as a `patch` change.

packages/backend-common/api-report.md

@@ -534,7 +534,7 @@ export const legacyPlugin: (
 export function loadBackendConfig(options: {
   logger: LoggerService;
   remote?: LoadConfigOptionsRemote;
-  additionalConfig?: AppConfig;
+  additionalConfigs?: AppConfig[];
   argv: string[];
 }): Promise<Config>;
 

packages/backend-common/src/config.ts

@@ -34,7 +34,7 @@ export async function loadBackendConfig(options: {
   logger: LoggerService;
   // process.argv or any other overrides
   remote?: LoadConfigOptionsRemote;
-  additionalConfig?: AppConfig;
+  additionalConfigs?: AppConfig[];
   argv: string[];
 }): Promise<Config> {
   const secretEnumerator = await createConfigSecretEnumerator({