Add service layer

[stefanschaller]

Add clustered POI layer support

Adds first-class support for clustered point-of-interest overlays on top of RouteMap, mirroring the declarative style of NoServiceAreaLayer.

Motivation

Consumers (e.g. route-ticket-app) need to render service points and border crossings as clustered, interactive markers. Previously this had to be hand-rolled against the underlying MapLibreMapController. This PR ports that pattern into route_map as a reusable API.

What's new

RouteMapPoiLayer + RouteMapPoiCategory
RouteMap(
  poiLayers: [
    RouteMapPoiLayer(
      identifier: 'poi',
      createSource: () async => GeojsonSourceProperties(data: ...),
      clusterTheme: RouteMapPoiClusterTheme(...),
      categories: [
        RouteMapPoiCategory(
          identifier: 'service-point',
          filter: ['==', ['get', 'type'], 'service'],
          markerPath: pinPath,
          svgIconPath: 'assets/icon.svg',
          theme: RouteMapIconTheme(background: blue, foreground: white),
          interactive: true,
          label: RouteMapPoiCategoryLabel(...),
        ),
      ],
    ),
  ],
  onPoiTapped: (event) => ...,
)