Support selecting individual submodules to update

[prein]

The goal is shorter checkout times for repos with many submodules. A particular job may need just one of many modules. It would be an equivalent of looping over

git submodule update --init $submodule_name

where a list of submodule_name would come from parameter passed to actions/checkout like so (for example):

  - uses: actions/checkout@v3
        with:
          submodules:
            only:
              - foo
              - bar

Ideally the implementation would have also the possibility for blacklisting the modules (naming the ones to not to update) with equivalent of -c submodule."contrib/foo".update=none

[nicola-lunghi]

+1 on this

[oerp-odoo]

For my case, I have monororepo, so specific projects only need some submodules, not all. Now its kind of redundant to checkout all of them (and its very slow as well).

[Garett-MacGowan]

+1