Add native location-provider override (setExternalLocation)

[trueshail]

Summary

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

Resolves #1085.

What's here

• iOS: ExternalLocationProvider.swift implements LocationProvider + HeadingProvider, registered via mapView.location.override(provider:).
• Android: ExternalLocationProvider.kt implements LocationProvider, registered via mapView.location.setLocationProvider(...).
• Both: a hand-written FlutterMethodChannel/MethodChannel exposing setExternalLocation/clearExternalLocation, kept isolated in LocationController/LocationComponentController so it stays easy to rebase onto future releases. This is deliberately not Pigeon-generated — the Pigeon input specs for this plugin aren't published, only the generated output.
• Dart: LocationSettings.setExternalLocation/.clearExternalLocation wrap the new channel.
• Docs: a README section under "User location" (matching the existing "Location puck" subsection's style), a doc-comment example on setExternalLocation itself, and a CHANGELOG entry.

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

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 Mapbox's own location engine ever being in the loop. Per #1085, this seems like a capability others have asked for too.

Testing

• flutter analyze clean on the touched files.
• Full test suite passing, including 3 new tests for the new channel (test/external_location_test.dart).
• Verified on real iOS simulator and Android debug builds of the example app.

Rebased onto current main from our original branch point at v2.29.0 — no conflicts.

[CLAassistant]

All committers have signed the CLA.