Set preview URL in configuration file

Hello! First, let me thank you for the great work on this plugin, it helped me a lot in previewing content using strapi.

I am opening this issue to check if I am missig something and it is possible to set the preview URL in a configuration file so it can be tracked in version control instead of setting it in the Strapi admin interface and being kept in the DB.

If it in fact is not possible, I would like to leave it as a suggestion, because it would remove any manual work from deploys (at least mine), and I suspect this could be the case for others too.

From quickly looking at the plugin code I imagine it would take adding some logic to check if a value exists in the strapi config object in the 'getPreviewUrl' function in the file 'src/services/preview.ts'. The config value could be set in the file 'plugins' config file in strapi or something similar.

Anyway, I could make a pull request if you're interested in the suggestion, and thanks again for your work on this plugin :smiley: