Add a custom container to the realtime plugin

[hansthen]

Add the container parameter from the leaflet realtime plugin to the Folium realtime plugin. So far, only tested with MarkerCluster. Also add tests and improved the documentation.

[hansthen]

pre-commit.ci autofix

[Conengmo]

Nice addition to this plugin! I've added 3 comments, hope you have time to address them. Afterwards we can get this merged!

[Conengmo]

This looks good Hans, the docstring is pretty clear so I hope users can work with this.

More longterm, we can add LayerGroup as an abstract base class to indicate something that will generate a L.LayerGroup javascript object. That way we can make the Python class hierarchy mimic the Leaflet class hierarchy.

Though that does make sense, I'm hesitant from a complexity perspective. Adding a core Folium class doesn't seem to weigh up to facilitating a single type hint for an optional argument in a single plugin. Having too many layers of class inheritance makes things more complicated to understand. So I think I'm fine with how it's solved now.

I'll go ahead and merge it now. Thanks for your contribution to Folium!