Geomap: Support coordinates across multiple data frames

[drew08t]

Currently, the location editor in Geomap has trouble selecting fields from multiple data frames. As a workaround, users can use the Concatenate fields transformation to combine all fields into a single frame, then carry on with using the location editor accordingly.

This is a good place to start: https://github.com/grafana/grafana/blob/main/public/app/features/geo/editor/locationModeEditor.tsx#L57