VRT Pixel Functions: Add function to evaluate arbitrary expression #14900

Job	Run time
build-linux-ubuntu-focal	1h 20m 24s
build-windows-conda	27m 55s
build-windows-msys2-mingw	14m 35s
build-windows-minimum	35m 49s
build-mac	13m 55s
build-windows-conda-csharp-only	0s
	2h 52m 38s

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