Add the ability to clone a specific commit of the Unreal Engine

[adamrehn]

This replaces the git clone command in the ue4-source Dockerfiles with an init/fetch/checkout workflow, which makes it possible to clone specific commits using their commit hash. This is tremendously useful when building and testing container images for currently unreleased versions of the Unreal Engine (5.0 at the time of writing) where stability of the Engine's build health is not yet guaranteed and it may be necessary to select specific commits in order to ensure a successful build.

The only potential issue with this change is that it alters the semantics of the clone_opts template option introduced in 46664f8, since the value of this option will now be passed to git fetch rather than git clone and the overlap in supported flags between these two commands is not 100%.

@slonopotamus what are your thoughts on this? Will the intended use case(s) for clone_opts still be satisfied by the flags supported by git fetch? If so, should we rename the template option to fetch_opts to reflect this new placement, or leave the current name as-is?