Add AVIF support for resize_image() function

[iWangJiaxiang]

The work for adding AVIF support for resize_image() has been finished, with pure rust crate.

With this feature, user may convert their images into avif format and the size will be reduced significantly.

However, the speed is slow in the developing env of GITHUB Code Space. I will take further tests on my machine.

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[iWangJiaxiang]

Is it necessary to add encoding speed arguement for resize_image()? @Keats

To compare time consumed to resize_image(), I tried command time zola build with deferrent encoding speed within my theme repo zola-theme-jiaxiang.wang, which has 16 images to be processed.

speed	real time	user time	sys time
1	5m54.865s	9m50.317s	0m0.821s
5	1m33.157s	2m30.512s	0m0.508s
6	1m26.289s	2m36.678s	0m0.442s
10	0m42.102s	1m14.762s	0m0.322s

The results tell the speed make significant sense for encoding.

Accoding to https://www.reddit.com/r/AV1/comments/qie51c/why_when_encoding_avif_images_with_avifenc_on/ , the speed 1-6 is good quality encoder mode and 7-10 is realtime encoder mode. Should we set 10 as default speed?