Use FetchContent to build ITK #986

Job	Run time
build-test-notebooks (windows-2022)	4m 21s
build-test-documentation (ubuntu-24.04)	44m 47s
build-test-cxx (windows-2022)	25m 54s
build-test-python-superbuild (macos-15)	22m 36s
build-test-cxx (macos-14)	17m 9s
build-test-python-superbuild (windows-2022)	23m 48s
build-test-notebooks (macos-14)	2m 48s
build-test-python-superbuild (ubuntu-24.04)	30m 39s
build-test-cxx (ubuntu-24.04)	20m 42s
build-test-notebooks (ubuntu-24.04)	2m 21s
	3h 15m 5s

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