Add an `INSTALL_BYPRODUCTS` option to `ExternalProject_Add` that can
be used to declare that files are `BYPRODUCTS` of the ExternalProject
install step.

This is often required by the Ninja generator to explicitly declare
dependencies. Previously, many users used `BUILD_BYPRODUCTS`, even if
their files were created by the install step, not the build step.

This commit essentially just copies the code for `BUILD_BYPRODUCTS`.

Fixes: #24120
Fixes: #23056