[Feat] Give map instance as input

[grillorafael]

Target Use Case

I have an instance of mapbox already being used in my app and when I was on an older version of react-map-gl I was create a context with the map instance and still using the Layer and Source components as normal.

After the upgrade I can't do this anymore so in my case where I have part of my app not written in React and part of it using React, I can't extend my existing map using react-map-gl as I was before.

Example:

<MapContext.Provider
      value={{
        map: {
          getMap: () => map,
        } as any,
        mapLib: mapboxgl as MapboxAsMapLib,
      }}
    >
.....

Where map was a prop of my component.

Proposal

Since I can't override the context anymore, it would be nice if the main Map component could receive an existing external map instance as an option

[grillorafael]

I'm thinking some component like this:

import {MapRef} from '../mapbox/create-ref';
import {MapLib} from '../types/lib';
import {MapContext} from './map';
import React from 'react';

export type ExternalMapProps = {
  mapLib: MapLib;
  map: MapRef;
  children?: React.ReactNode;
};

export default function ExternalMap(props: ExternalMapProps) {
  return (
    <MapContext.Provider value={{mapLib: props.mapLib, map: props.map}}>
      {props.children}
    </MapContext.Provider>
  );
}