Report form submissions via TurboNavigatorDelegate

[zoejessica]

@joemasilotti I've added support for reporting form submissions via TurboNavigatorDelegate here, since it's something we needed for internal work. @jayohms let me know that it's functionality they expose on the Android side, so it seems like a good candidate for us here too.

[jayohms]

Thanks @zoejessica! Let's expose both formSubmissionStarted and formSubmissionFinished to apps. It'll also be common for apps to use this to display a progress indicator (such as in the top app bar) when forms are being submitted and waiting for the network response.

[olivaresf]

Looks good 👍🏻. I feel this is a good example of us slowly moving functions into the public API as needed.

[joemasilotti]

+1 on adding the corresponding callback when the form completes. Otherwise, this looks good to me!

It also calls out that we need to document what happens when a form submission happens by default - clearing the snapshot cache of the main session.

[jayohms]

How difficult is it to add a progress indicator to the demo app between the started/finished events? The turbo-android demo app displays a progress indicator, which is a nice touch on modal/form screens and it'd show other devs how to do this without extra documentation.

[joemasilotti]

@jayohms I added the other side of the form submission.

[joemasilotti]

Going to merge this into the turbo-navigator branch now. Thanks folks!