[cloud_firestore] Add custom configuration option of experimentalForceLongPolling

[alexagat]

After deploying a Flutter web application many users report not being able to use it due to failing requests to read/write data. Specifically the gRPC network requests to https://firestore.googleapis.com/google.firestore.v1.Firestore/Listen/channel?... are the ones failing.

All of the users experiencing this issues are on a corporate network or have some Firewall layer enabled. This same issue occurs with non-Flutter web applications built in Angular/React using the firebase-js-sdk SDK, but the javascript sdk allows for setting an option of experimentalForceLongPolling which seems to help.

Documentation on JS SDK FirebaseFirestore options: https://firebase.google.com/docs/reference/js/firebase.firestore.Settings#experimentalforcelongpolling

Related issue against the JS SDK: firebase/firebase-js-sdk#1674 firebase/firebase-js-sdk#2526

Unfortunately, experimentalForceLongPolling does not appear to be an option for the FlutterFire SDK. Can it be added?

[TechnoMarc3]

Really nice, that after two years they didnt add this IMPORTANT feature, good job!

[RizanPSTU]

Any way to add this?