KmlLayer: Add mocks for maps.KmlLayer

 **Is your feature request related to a problem? Please describe.**
No, just additional functionality

 **Describe the solution you'd like**
Mocks should exist for the maps.KmlLayer class defined here: https://developers.google.com/maps/documentation/javascript/reference/kml

 **Describe alternatives you've considered**
None

 **Additional context**
PR will be opened shortly
