feat(server): random assets API (#4184)

* feat(server): get random assets API

* Fix tests

* Use correct validation annotation

* Fix offset use in query

* Update API specs

* Fix typo

* Random assets e2e tests

* Improve e2e tests

server/test/repositories/asset.repository.mock.ts

@@ -11,6 +11,7 @@ export const newAssetRepositoryMock = (): jest.Mocked<IAssetRepository> => {
     getWithout: jest.fn(),
     getByChecksum: jest.fn(),
     getWith: jest.fn(),
+    getRandom: jest.fn(),
     getFirstAssetForAlbumId: jest.fn(),
     getLastUpdatedAssetForAlbumId: jest.fn(),
     getAll: jest.fn().mockResolvedValue({ items: [], hasNextPage: false }),