Basemap config with wmts.getCapabilitiesUrl and optional
wmts.layer, wmts.tileMatrixSet, wmts.format.
Optionalsignal: AbortSignalOptional AbortSignal for cooperative cancellation.
The resolved XYZ tile URL, or null on failure or cancellation.
Fetches WMTS GetCapabilities XML, resolves the XYZ tile URL template, and caches the result for subsequent calls with the same GetCapabilities URL.