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
Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win7-windows-release-ninja

00:24:22

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:53:22

passed cmake-hythloth-linux-release-lint-iwyu-tidy

00:02:44

passed cmake-moonshade-osx-debug-ninja

00:11:10

passed cmake-moonshade-osx-release-makefiles

00:12:34

passed cmake-terfin-windows-release-vs2017

01:22:42

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