`update_code` recipe - 3rd option `update_code_strategy` that loads submodule

[AWqxKAWERbXo]

This adds the ability to use submodules with update_code set to clonse_submodule while staying backwards compatible.

Issue: https://github.com/deployphp/deployer/issues/2802

Using submodules were possible in Deployer 6 but not 7 and could be reintroduced like this.

• [ ] Bug fix
• [x] New feature?
• [ ] BC breaks?
• [ ] Tests added?
• [ ] Docs added?

Please, regenerate docs by running next command: $ php bin/docgen