feat: image widget insert from url should be optional

[Edald123]

Summary
Fixes: Image widget "Insert From URL" should be optional as requested here: #5558

Test plan
By puting the option choose_url as true:

We see this in the cover image section when creating a new post:

By putting false:

We see that the option of Insert From URL does not appear:

A picture of a cute animal (not mandatory but encouraged)

[erezrokah]

This looks great @Edald123. I made some minor changes to the docs and to return null instead of an empty string

[madsem]

Huzzah, thanks @Edald123 & @erezrokah

[Edald123]

Great, Thank you!