iOS Widget Extension on EAS Build

you can check GitHub - OneSignal/onesignal-expo-plugin: The OneSignal Expo plugin allows you to use OneSignal without leaving the managed workflow. Developed in collaboration with SweetGreen.
they are adding notifications service extensions in that plugin

support for generating credentials was added recently (no docs yet other than PR description [eas-cli] generate credentilas for iOS app extensions by wkozyra95 · Pull Request #1039 · expo/eas-cli · GitHub) (or you could use credentilas.json)
expo does not support entitlements for multiple targets, so your extension would get the same one as the main target