Ability to override the default marker icon

[JarrydLong]

Is your feature request related to a problem? Please describe.
Add in support for custom marker icons.

Describe the solution you'd like
Add in a filter with a level attribute that allows you to change the marker icon based on the marker's level or even shortcode attributes.

Describe alternatives you've considered
No possible alternatives as there needs to be JS changes

Additional context
N/a

[andrewlimaza]

This would be really cool. We do need to discuss this a bit more:

1. Would we support something similar like Member Badges? Set a marker icon per membership level?
2. Add in a user meta option to override the icon on a per user level?

[MaryOJob]

Moderators Only:
Another Request #472522

[MaryOJob]

Another Request (Moderators Only): #474795

[ipokkel]

Another Request (Moderators Only): #480525

[tridentmarketinguk]

Another request