Forms: Ability to autocomplete email address of logged-in users

### What

Add the ability to pre-fill input fields on the Jetpack Form block with the logged-in user information, like their email address.

### Why

The user on 6088200-zen have some forms that should be filled out by their site members. My understanding is they need to do that routinely, so having the option to pre-fill as many inputs as possible would be helpful.

### How

I see this is already implemented in the "Subscribe" block. Maybe we could replicate the same to the Form block.

Begin with the Jetpack Form block and compare the relevant user-prefill behavior in the Subscribe block. Trace how logged-in user information is handled and identify the surrounding tests; done means the Form block can pre-fill the email field for logged-in users while preserving the existing behavior for other users.

Written by the indexing model from the issue text.