feat(street-level): provider exports and a Mapillary Graph endpoint for hosts (#1324)

* feat(street-level): export the KartaView and Panoramax providers

A host that composes Street Level with its own provider list can already
import the Mapillary and Google Street View providers; KartaView and Panoramax
were reachable only through the default list, which needs a Mapillary source.
Exporting them lets a host offer the keyless providers without Mapillary.

* chore: changelog entry for the Street Level provider exports

* feat(street-level): send Mapillary Graph lookups to a source endpoint

A Mapillary source can take a graphEndpoint, a Graph API base that adds
the token itself. Its sequence lookups go there through the source's
transport, and the photo viewer uses MapillaryJS's stock GraphDataProvider
at that endpoint, so a host that proxies Mapillary needs no client token in
the page. Without it, behaviour is unchanged: direct Graph API calls with
the client token.

src/app/layers/streetLevel.test.mjs

@@ -72,3 +72,17 @@ test('the defaults are Mapillary (given its source), Street View, KartaView and
   // Without the application's Street Level source there are none.
   assert.deepEqual(createApplicationStreetLevel({}).providerIds, []);
 });
+
+test('every built-in provider is exported, so hosts can choose their own list', async () => {
+  const google = await import('gods-eye-view/layers/street-level/google');
+  const kartaview = await import('gods-eye-view/layers/street-level/kartaview');
+  const panoramax = await import('gods-eye-view/layers/street-level/panoramax');
+  const layer = createApplicationStreetLevel({
+    providers: [
+      google.createGoogleProvider({ getApiKey: () => null }),
+      kartaview.createKartaViewProvider(),
+      panoramax.createPanoramaxProvider(),
+    ],
+  });
+  assert.deepEqual(layer.providerIds, ['google', 'kartaview', 'panoramax']);
+});