Support --repo, --no-repo, and autoinit in dstack apply

[r4victor]

Closes #2072

This PR improves the repos and dstack apply UX:

• Allows specifying a repo for the run via -P/--repo. The repo can be a local path or a remote Git repo URL. Previously, dstack always used the current directory as a repo and required it to be initialized.
• If the repo is specified but not initialized, dstack apply will init the repo. dstack apply accepts the same parameters as dstack init to init the repo. This is primarily useful when working with remote repos without cloning them locally.
• Allows specifying --no-repo if no repo is needed for the run. In this case, the run repo directory (/workflow) will be empty.

If the repo is not specified, dstack apply requires the directory to be initialized as before.