Add CLI option to specify additional packages for internal Cygwin at opam init

[dra27]

This is to mitigate #5833, principally for CI, as it would allow a workflow which knows it's creating a mingw-w64 switch to speed up the init process by adding mingw64-x86_64-gcc-core or mingw64-i686-gcc-core to the installation when it is created (indeed, a CI workflow might choose to add additional depext packages).

At a rough punt, this could be either a dedicated --cygwin-internal-packages=package1,package2 but, given that that would need checking as it would conflict with --cygwin-local-install, it could just be made as an optional argument for --cygwin-internal-install?

In principle, we might want to consider having general maintenance commands for the internal Cygwin installation (although it's possible to do that by hand simply by running the setup program against the root), but that's a much bigger feature and the whole thing is still experimental anyway.