Added a NET_CORE define to allow us to have changes side by side.
The AWS S3 changes are required due to us requiring to upgrade the S3 assembly version to get net core support (which made all methods async).
The ACL checks for files are not available in the system libraries of net core, as such the api is a bit different.
AutomationToolLauncher now just spawns a subprocess when used in netcore, as netcore does not support custom AppDomains and shadow copying. We will generally need to revisit this for netcore as this whole feature of building the source for UAT in UAT is not really possible.
To enable this set environment variable "UE_USE_DOTNET=1", note that with netcore all applications change their output path so this will likely break a bit of tooling when enabled.
#rb ben.marsh
[CL 14572339 by Joakim Lindqvist in ue5-main branch]
