Add optional serde feature for image::ImageFormat #1680

Job	Run time
Fuzz targets (cargo-fuzz)	54s
cargo-deny	19s
build_benchmarks	47s
Fuzz targets (afl)	2m 7s
cargo-semver-checks	29s
clippy	1m 58s
public_private_dependencies	24s
rustfmt	12s
test_avif_decoding	1m 26s
test_other_archs (powerpc-unknown-linux-gnu)	2m 41s
test_features	32s
test_other_archs (i686-unknown-linux-gnu)	2m 1s
test_toolchains (1.70.0)	48s
test_features (default)	1m 50s
test_toolchains (nightly)	1m 46s
test_features (rayon)	29s
test_toolchains (beta)	1m 33s
test_features (avif)	1m 3s
test_features (bmp)	30s
test_features (dds)	29s
test_features (exr)	42s
test_features (ff)	31s
test_features (gif)	32s
test_features (hdr)	29s
test_features (ico)	40s
test_features (jpeg)	30s
test_features (pcx)	23s
test_features (png)	28s
test_features (pnm)	26s
test_features (qoi)	28s
test_features (tga)	22s
test_features (tiff)	32s
test_features (webp)	28s
	28m 49s

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