Make it possible to render a `PostPicker` with the modal open

### Description

(Or image picker, term picker, etc.)

I would like for it to be possible to configure a `PostPicker` so that the modal opens immediately when the component renders and a post isn't selected, rather than having to hit the "Select" button.

### Use Case

I have a block with a "setup" state that requires the user to select a post. Currently, after the user adds the block to the post, they have to find and click the "Select" button to open the search modal, even though there's nothing else for the user to do until the post is selected. The user experience would be improved by displaying the modal right away.