build(plugin-keychain-memory-wasm): upgrade OpenAPI generator to v6.6.0

[solo-daemon]

fix #2181 steps to reproduce :

cd packages packages/cactus-plugin-keychain-memory-wasm/ npm run generate-rust-server then make changes related to PrometheusExporterMetrics Also, the server expects Option<> type response on api calls but the generated code does not supports that so made changes to support that.