feat(extensions): with git operations

[kjuulh]

Quite simply implements fetch and clone to make sure we can pull a registry

Git repository is the only registry implemented for now, and is run like this:

shuttle ext update --registry git=github.com/lunarway/shuttle-extensions.git

or

export SHUTTLE_EXTENSIONS_REGISTRY="git=github.com/lunarway/shuttle-extensions.git"
shuttle ext update

Builds on: #221