Feat: Add configurable events to interact with frontend app

[amit-s19]

Is your feature request related to a problem? Please describe.

When integrating Enketo Forms with frontend applications made on React, Angular, plain html css etc it becomes for these apps to interact with the form directly.

Describe the solution you'd like

Events should be provided by default like onFormChange, onSubmit, onSubmitSuccessful, onFailure, onFileUpload, etc which would allow the frontend application to interact with the enketo form.

Describe alternatives you've considered

We've added events manually in controller-webform.js to suit our needs. For example, an event listener was added to events.XFormsValueChanged to listen to form changes event and then data was broadcasted to the frontend application.

  document.addEventListener(events.XFormsValueChanged().type, async (e) => {
        /*
        * Perform actions on event change
        * /
        
       // Broadcast back to frontend
        await formController.broadcastFormDataUpdate(form.getDataStr(), {});
}

Note : This feat/issue is linked to previous issues(s) #940, #939

[lognaturel]

Related to enketo/enketo-express#454

[lognaturel]

Discussion can continue at #1314