Add support for external repos

[bpmct]

Some contributors may wish to maintain their templates/modules in their own repos. Can we add support for this, with the caveat that we cannot guarantee the safety or effectiveness of these

[DevelopmentCats]

This should be possible in the sense that if someone fork's our registry, they can point the module source to their module in their repo.

This is similar to how I test modules from branches or forks currently: git::https://github.com/coder/registry.git//registry/coder/modules/claude-code?ref=your-branch-name