feat: support digests for custom datasource

Support digets for custom datasource.
The getDigest method needs to exist on the custom datasource for
renovate to use the digest provided from the main versions/releases
endpoint. If the versions response always includes the digest, the
getDigest method doesn't actually need to be implemented.

https://github.com/renovatebot/renovate/blob/14b67888307e2a83219e4bb90f7f0f71f5e608bd/lib/workers/repository/process/lookup/index.ts#L446

See renovatebot#26299

lib/modules/datasource/custom/index.ts

@@ -2,7 +2,7 @@ import is from '@sindresorhus/is';
 import jsonata from 'jsonata';
 import { logger } from '../../../logger';
 import { Datasource } from '../datasource';
-import type { GetReleasesConfig, ReleaseResult } from '../types';
+import type { DigestConfig, GetReleasesConfig, ReleaseResult } from '../types';
 import { fetchers } from './formats';
 import { ReleaseResultZodSchema } from './schema';
 import { getCustomConfig } from './utils';
@@ -57,4 +57,11 @@ export class CustomDatasource extends Datasource {
       return null;
     }
   }
+
+  override async getDigest(
+    config: DigestConfig,
+    newValue?: string
+  ): Promise<string | null> {
+    return await Promise.reject('Not implemented');
+  }
 }