FetchContent: Give access to the terminal for download and update
A main scenario where this is needed is when a git operation needs the password to a private key and asks for it on the console. Without this change, such operations can appear to hang indefinitely with no prompt if QUIET is in effect (which it is by default).
Another scenario this addresses is when progress of a download or update should be shown. Without this change, all such progress is buffered with some generators and will only be shown at the end, which defeats the purpose of logging any progress to begin with.
Relates: #18238 (closed)