Providing Custom Gazetteer File

**Why is this needed**: This allows the usage of heatmap together with a custom gazetteer file. An example use case would be datacenter locations, where our query has values for each datacenter and the provided gazetteer file would provide their mappings and coordinates. **What would you like to be added**: Basically a way to to configure `public/app/features/geo/editor/GazetteerPathEditor.tsx` via a config file or env variables. Mounting it and replacing it with a new file doesn't do anything as I believe Grafana is using the pre-compiled version of this file. Relevant: https://community.grafana.com/t/custom-gazetteer-file/106274 **Who is this feature for?** Users of the Geomap plugin I could contribute to implement this, but please let me know if this is of any interest and how you would like to have it implemented.