Enhancement: Gallery Block Hooks

### Impacted plugin

Jetpack

### What

Gallery blocks could benefit from a hook to change the default behaviour of image links in the gallery

### How

Presently there do not appear to be any hooks that would enable one to change the default behaviour of image links in the gallery.

The default is presently none and it would be very helpful to have a hook that would enable users to set custom default behaviours ex: default all image links to open full image etc.

Start by locating the Jetpack Gallery block implementation and the code that determines default image-link behavior, then inspect whether related hooks already exist. Define the hook's expected customization scope and verify that users can set alternatives such as opening the full image, with tests covering the resulting gallery behavior.

Written by the indexing model from the issue text.