Limit the Link field to certain domains
You can use the Field Validation module


  This module adds an extra tab to each field instance, allowing you to specify validation rules for your field instances.


From there you could a custom PHP function/regular expression to match the URLs against the field value.

This will only editable by admin in the sense that they can also edit the PHP code/regular expressions in the field validation rule. I'm not aware of a specific module that will do everything that you want without any customisation.