Cloud API: support composes/<id>/manifests endpoint for non-koji builds

Support the composes/<id>/manifests API endpoint for non-koji builds.
The endpoint will have to anyway handle `osbuild` job results once Koji
composes will start using `osbuild` job type for builds.

The endpoint previously contained a bug. If the `osbuild-koji` job had
an empty manifest attached as a static job argument (this is the default
type value), then this empty manifest was added to the endpoint
response. Since Cloud API uses the depsolve and manifest jobs, the
actual manifest was never attached to the job as a static argument. As a
result, the endpoint was always returning an empty manifest for any koji
compose. Fixing this required also adjusting unit tests, which was
relying on the buggy behavior.

Extend the unit test testing a successful compose to test the logs
endpoint.

internal/cloudapi/v2/v2_test.go

@@ -646,6 +646,19 @@ func TestComposeStatusSuccess(t *testing.T) {
 			}
 		]
 	}`, jobId, jobId))
+
+	test.TestRoute(t, srv.Handler("/api/image-builder-composer/v2"), false, "GET", fmt.Sprintf("/api/image-builder-composer/v2/composes/%v/manifests", jobId), ``, http.StatusOK, fmt.Sprintf(`
+	{
+		"href": "/api/image-builder-composer/v2/composes/%v/manifests",
+		"id": "%v",
+		"kind": "ComposeManifests",
+		"manifests": [
+			{
+				"pipeline": {},
+				"sources": {}
+			}
+		]
+	}`, jobId, jobId))
 }
 
 func TestComposeStatusFailure(t *testing.T) {