#2899 Add ANI decoder support

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Description

implements AniDecoder for ImageSharp, and encoder if the pr approved

Comment

Since the ANI file may contain a two-dimensional array of ImageFrames, I flattened the original ImageFrames as follows

I have kept all Metadata as much as possible, but this may result in a structure that is not intuitive, and may require further discussion with you) reserved the possibility of future modifications

ANI is also a RIFF file, so I referenced RiffHelper, perhaps we should move it out of the WEBP namespace

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