Add option to fetch only specyfic submodules

[andr729]

Currently there is only option to checkout al submodules or none.
It would be useful to be able to specify which submodules to checkout.

It my case the reason is that I only need to checkout submodules that don't require authorization. But because action is getting all submodules I still need to provide it with some token/key.

[kwshi]

linking #1095 to consolidate discussion since it sounds like the same issue