Add ability to save autofill data and trigger autofill on form elements

Problem

Currently there is no way for developers to automate autofill in browsers and thus they can't test their HTML with the browser's autofill behaviour. Adding such an API would allow such testing and enable developers to write automated autofill tests (e.g. WPT).

Proposal

We are proposing to add 2 new end points to webdriver:

Endpoint /session/{session id}/autofill/save

This endpoint can be used to save an address in the autofill manager of the browser which is necessary to set up any automated tests. It should take a JSON object as input with the keys being the field name and the values being the actual value for that field.

Endpoint /session/{session id}/autofill/trigger

This endpoint will be used to trigger the autofill behaviour on a specific input field, similar to how a user would do it when using the browser manually. It should take the field's id attribute as parameter to specify which input field autofill should be triggered on.

Start by reviewing the WebDriver endpoint definitions and the HTML autofill field specification linked in the issue. Define how the /session/{session id}/autofill/save and /session/{session id}/autofill/trigger endpoints should behave, then validate the result with automated autofill tests such as WPT.