OGRLayer::GetArrowStream(): add a DATETIME_AS_STRING=YES/NO option #6818

Job	Run time
build-linux-ubuntu-focal	1h 14m 30s
build-windows-conda	27m 5s
build-mac	8m 38s
build-windows-msys2-mingw	12m 32s
build-windows-minimum	29m 20s
build-windows-conda-csharp-only	0s
	2h 32m 5s

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