Allow formatting the entire preview url and not just the path

**Is your feature request related to a problem? Please describe.**
I need to format the preview link given from the target_url statuse api.
Indeed, my deploy provider is giving it in the wrong format.

**Describe the solution you'd like**
A simple hook that would allow me to edit the deploy preview url

**Describe alternatives you've considered**

**Additional context**
Could be done like https://github.com/netlify/netlify-cms/pull/3812