[Feature] Support `repos` in run configurations (and deprecate `dstack init`)

[peterschmidt85]

1. Introduce the repos property (similar to files)

repos:
  - .:/workflow

or

repos:
  - .

or

repos:
  - repo_path:  .
     path: .
     identity_key: ~/.ssh/id_rsa

or

repos:
  - repo_path:  .
     identity_key: ~/.ssh/id_rsa

or

repos:
  - repo_path:  .
     token: {env. GH_TOKEN}

Note, if the repo is remote, dstack will try to use the default Git credentials (as it does now) using ~/.config/gh/hosts.yml or ~/.ssh/id_rsa.

A repo path can be also a GitHub URL (as it's supported now):

repos:
  - repo_path:  https://github.com/huggingface/open-r1

or

repos:
  - https://github.com/huggingface/open-r1:/workflow

2. Introduce ssh_config.identity_file
3. Deprecate dstack init