GetCropUrl(): Improved support for frames (and possibly make it async?)

[RachBreeze]

On gifs with a large number of frames and are seeing memory issues when they are cropped. The issue did not occur with Umbraco 8 and earlier sites. It is ocuring with Umbraco 13.

Image Sharp have been great and responded with SixLabors/ImageSharp#2848 (comment)

There are two requests from there:

• Is it possible to make GetCropURL async, so large images dont become a blocking request
• Is it possible to configure the number of frames to use

Thank you