Add native location-provider override (setExternalLocation)

[trueshail]

Summary

Adds a way to drive the map puck from an externally-supplied location instead of the default GPS-only location engine — a custom LocationProvider/HeadingProvider on both mobile platforms, wired through a small setExternalLocation/clearExternalLocation API on the location settings.

Resolves #1085.

Updated for v3: this branch has been ported from the pre-v3 flat layout to the federated package structure introduced in #19427.

What's here

• mapbox_maps_flutter_platform_interface: LocationSettingsPlatformInterface gains setExternalLocation/clearExternalLocation.
• mapbox_maps_flutter: LocationSettingsManager forwards both to the platform implementation.
• mapbox_maps_flutter_mobile (Dart): LocationSettings backs them with a hand-written MethodChannel. This is deliberately not Pigeon-generated — the Pigeon input specs for this plugin aren't published, only the generated output — so the channel is kept isolated from generated code.
• mapbox_maps_flutter_mobile (iOS): ExternalLocationProvider.swift implements LocationProvider + HeadingProvider, registered via mapView.location.override(provider:).
• mapbox_maps_flutter_mobile (Android): ExternalLocationProvider.kt implements LocationProvider, registered via mapView.location.setLocationProvider(...).
• mapbox_maps_flutter_web: throws UnsupportedError. GL JS's GeolocateControl owns the puck position and exposes no location-provider override, so there is nothing to override.
• Docs: doc comments on the facade (including a usage example) and a CHANGELOG entry. Happy to add prose to the docs site separately — the v3 README is a quickstart and no longer carries per-feature guide sections.

The override activates lazily on the first setExternalLocation call; clearExternalLocation restores the platform default provider (GPS). Until either is called, puck behavior is unchanged — this is purely additive.

floor is applied on iOS only, whose native Location type carries it. Android's LocationConsumer API has no floor concept, so it's dropped there; this is documented on the API.

Usage

// Start driving the puck from an external source.
mapboxMap.location.setExternalLocation(
  latitude: 37.775,
  longitude: -122.418,
  heading: 90.0,
  accuracy: 5.0,
);

// Later, hand the puck back to the platform's default provider (e.g. GPS).
mapboxMap.location.clearExternalLocation();

Motivation

Useful for apps that need to drive the puck from a custom positioning source — e.g. an indoor-positioning SDK while inside a mapped venue, falling back to GPS outdoors — without the platform's own location engine ever being in the loop. Per #1085, this is a capability others have asked for too.

Testing

• flutter analyze across the workspace reports no new issues: 247 before and after this change, all pre-existing.
• All four package suites pass (286 tests), including 3 new channel tests in mapbox_maps_flutter_mobile/test/external_location_test.dart and 3 new delegation tests in mapbox_maps_flutter/test/location_settings_test.dart.
• The native sources are unchanged from the version previously verified on real iOS simulator and Android debug builds of the example app; LocationComponentController.kt and LocationController.swift are byte-identical between the 2.x and v3 trees, so the port moved them without modification. The example app has not been rebuilt against v3 as part of this port — worth a CI run before merge.

Note on API shape

I've added the two methods directly to LocationSettingsPlatformInterface, which makes them required for any implementer. If you'd prefer them optional (default implementations that throw, so out-of-tree platform implementations don't break), say the word and I'll restructure.

[CLAassistant]

All committers have signed the CLA.