ImageServiceRaster should support format similar to ImageryLayer

[dev4567]

# Context

We use `ImageServiceRaster` with `RasterLayer` along with the MosaicRule

# Issue

There is no way to provide `format` parameter which leads to ImageServer request with "lerc" format, which causes transparency issues. The request path & query is smth like: /ImageServer/exportImage?format=lerc&...

# Request

Provide `format` or `preferredImageFormat` (similar to WmtsLayer) so we could decide which tile format we want to obtain from the image server.

[XuemingWu]

Hi @dev4567,

Thanks for reporting the issue and sharing your request. 

Curious to know more about your issue. Is transparency the main reason you want to work with other raster format? 

ImageServiceRaster exports data from the image service using "lerc" format as is an efficient lossy compression method. This allows to apply raster functions to the ImageServiceRaster at the client-side like Raster created from local datasets.  A possible workaround of the transparency issue could be applying a  mask raster function to your ImageServiceRaster.

If you only use ImageServiceRaster with RasterLayer to visualize imagery data, you can use ArcGISTileLayer with the image service that also supports cache tiles. 

Hope this helps!