Geomap: Allow layer selection on dashboard

**Why is this needed**: We monitor assets that have properties which can be sliced and diced in different interesting ways and visualized on a map. Different teams are interested in different things. Unless I missed something, right now we have to create N different dashboards each showing a different layer. I was hoping I could just have the N different data layers and let the user chose which one they want to see but the doesn't seem to be an option. That way we only have to maintain one dashboard. This also makes it much easier to keep styling consistent. **What would you like to be added**: A checkbox `[ ] Allow User Data Layer Selection` which makes an overlay available on the panel that allows users to select the Data layers they want to see. Should also be possible to specify via URL parameter so they can bookmark they desired view. Similar suggestion: https://community.grafana.com/t/hiding-layers-in-the-geomap-plugin/82162 **Who is this feature for?** Different teams in our company.