Also added CancelAsyncBuild and clarified the comments on how async builds work.
#rb Zousar.Shaker
#rnx
[CL 16406496 by Devin Doucette in ue5-main branch]
IBuildFunction and the associated build context types define the interface for new derived data build functions. The inputs to a build function are referenced by FBuildDefinition. This only includes the functionality required to implement and register a build function, and does not include the functionality required to execute a build starting from a build definition.
#rb Zousar.Shaker
#rnx
[CL 16278733 by Devin Doucette in ue5-main branch]
