Better support evalscriptUrl in BYOCLayer

[zcernigoj]

Call fetchEvalscriptUrlIfNeeded in BYOCLayer.getMap(), same as in AbstractSentinelHubV3Layer, so that when only an evalscriptUrl is provided to the BYOCLayer.getMap(), it gets the evalscript automatically.

This is useful for BYOC for visualizations with long evalscripts.

Next step is to improve supportsApiType() function in AbstractSentinelHubV3Layer.ts and BYOCLayer.ts to allow ProcessAPI when evalscriptUrl is provided instead of evalscript. And add this support for getStats() as well.