Correct typing annotations in Realtime

1. Realtime superclass in leaflet is GeoJson (which is a subclass
of featuregroup). In Folium I cannot make Realtime a subclass of
GeoJson since GeoJson requires features to be present before
rendering. I made it a subclass of FeatureGroup to more clearly
document that features can be added to a Realtime layer.

2. The container parameter for Realtime cannot just be any `L.Layer`.
It must be a `FeatureGroup` or something that allows adding features.

I created type annotations to reflect this on the Folium side.

folium/plugins/realtime.py

@@ -1,14 +1,14 @@
 from typing import Optional, Union
 
-from branca.element import MacroElement
 from jinja2 import Template
 
 from folium.elements import JSCSSMixin
-from folium.map import Layer
+from folium.features import GeoJson
+from folium.map import FeatureGroup
 from folium.utilities import JsCode, camelize, parse_options
 
 
-class Realtime(JSCSSMixin, MacroElement):
+class Realtime(JSCSSMixin, FeatureGroup):
     """Put realtime data on a Leaflet map: live tracking GPS units,
     sensor data or just about anything.
 
@@ -109,7 +109,7 @@ def __init__(
         get_feature_id: Union[JsCode, str, None] = None,
         update_feature: Union[JsCode, str, None] = None,
         remove_missing: bool = False,
-        container: Optional[Layer] = None,
+        container: Optional[Union[FeatureGroup, GeoJson]] = None,
         **kwargs
     ):
         super().__init__()