refactor(vcs): VCS configuration to support common and custom attributes

[wkl3nk]

While VCS implementations are already plugins, they are not yet configurable. VCS implementations require common configurations (e.g., revision, recursive) and should support VCS-specific configurations that are unique to each VCS.

This refactoring keeps the common configuration attributes as they are, while VCS-specific configurations are stored generically in an options attribute.

The change also adds VCS-specific configuration options for Git: The strategy for checking out repositories with submodules now allows to only check out the first layer of submodules.

For more details, please have a look at the respective commits.

Fixes #8556.