This PR introduces a new IsochroneMapProvider class to compute and draw isochrone polygons on a Google Map. It supports multiple durations, transport modes, and color schemas, with a loading indicator callback to track rendering progress.

Features

• Isochrone computation:
  Uses Google Directions API to estimate travel time for radial points around an origin, refining distances with a binary search to form accurate isochrone polygons.

• Drawing order and colors:
  Polygons are drawn from outermost to innermost with a green-to-red gradient (green = shortest duration, red = longest) for intuitive visualization.

• Hole clipping between isochrones:
  Inner polygons are added as holes inside outer polygons to prevent visual overlap and create distinct rings.

• Loading listener:
  Callbacks notify when loading starts and finishes, enabling UI feedback such as progress spinners.

• Transport mode flexibility:
  Supports bicycling, walking, driving, and transit modes via Google Directions API.

• Smooth polygons:
  Chaikin smoothing is applied to polygon edges for improved appearance.