Add 'uet internal engine-checkout' command #88

Job	Run time
Test StringEnum Trimmed	38s
Timestamp	12s
Build AutoDiscovery	54s
Build Mac Logging	36s
Build macOS Pass 1	7m 8s
Build and Test Libraries on Windows	8m 33s
Build Windows Pass 1	7m 48s
Build Libraries on macOS	6m 31s
Build macOS Shim	49s
Build macOS UEFS	2m 3s
Build Windows UEFS	3m 4s
Build Windows Shim	1m 4s
Build macOS Pass 2	5m 22s
Build Windows Pass 2	7m 58s
Publish UEFS Container	0s
Publish to GitHub	0s
Publish to NuGet	0s
	52m 40s

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