Use FetchContent to build ITK #992

Job	Run time
build-test-notebooks (windows-2022)	5m 0s
build-test-notebooks (ubuntu-24.04)	2m 28s
build-test-python-superbuild (macos-15)	23m 32s
build-test-python-superbuild (windows-2022)	24m 0s
build-test-python-superbuild (ubuntu-24.04)	27m 14s
build-test-documentation (ubuntu-24.04)	39m 22s
build-test-notebooks (macos-14)	2m 41s
build-test-cxx (ubuntu-24.04)	21m 12s
build-test-cxx (windows-2022)	25m 12s
build-test-cxx (macos-14)	15m 31s
	3h 6m 12s

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