Restructure CI to use reusable steps, and add new checks #2290

Job	Run time
Linux.1: Release / steps	14m 11s
Linux.10: vfx2021 / steps	13m 30s
Linux.2: Debug / steps	24m 18s
Linux.3: static / steps	14m 26s
Linux.4: threads=OFF / steps	13m 39s
Linux.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal deflate / steps	4m 15s
Linux.6: custom namespace / steps	14m 29s
Linux.7: clang / steps	14m 14s
Linux.8: vfx2023 / steps	13m 49s
Linux.9: vfx2022 / steps	13m 37s
macOS.2: Debug / steps	18m 1s
macOS.1: Release / steps	7m 16s
macOS.3: static / steps	7m 36s
macOS.4: threads=OFF / steps	7m 46s
macOS.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / steps	1m 22s
macOS.6: macos-13 / steps	19m 34s
Windows.1: Release / steps	13m 15s
Windows.2: Debug / steps	40m 10s
Windows.3: static / steps	12m 48s
Windows.4: threads=OFF / steps	13m 1s
Windows.5: pkgconfig=OFF, examples=OFF, tools=OFF, docs=OFF, internal imath+deflate / steps	1m 46s
Windows.6: vfx2022 / steps	13m 25s
	4h 56m 28s

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