vehicle-rental-overlay: map marker icon slots (custom map icons for e-scooters, etc...)

[fpurcell]

Need the ability to customize map (marker) icons for things like e-scooters, bike rentals, etc...

Current customization is limited to fill colors of fixed marker icons (circles and peg map markers):
https://github.com/opentripplanner/otp-ui/blob/master/packages/core-utils/src/types.js#L74

NOTE: transit-vehicles has slots that allow fairly broad level of marker icon customization. https://github.com/opentripplanner/otp-ui/tree/master/packages/transit-vehicle-overlay/src/components/markers