Decouple maps plugin and vis_type_vega plugin from maps_legacy plugin

[nreese]

maps_legacy plugin contains lots of functionality. Some used by every mapping solution in Kibana. Other used specifically by tile_map and region_map plugin.

maps plugin and vis_type_vega plugin need to be decoupled from maps_legacy plugin making it possible to prune maps_legacy, tile_map, and region_map plugins without effecting maps plugin and vis_type_vega plugin.

I propose making a new plugin, maps_common under src/plugins that would contain only the common functionality used by all mapping plugins.

• All mapping kibana.yml configurations from maps_legacy

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)