[Feature] Support proxy_jump under ssh_config with fleet YAML config

[peterschmidt85]

Setup example:

1. A login-node that can access SSH into compute-node-1 and compute-node-2.
2. On a server node, run dstack server
3. Create a fleet config:

type: fleet
name: private-fleet

placement: cluster

ssh_config:
  hosts:
    - compute-node-1
    - compute-node-2
   proxy_jump: # also can be defined per host; (or it can be a hostname instead of an object)
     hostname: login-node
     identity_file: ~/.ssh/private_key

4. The dstack apply and dstack attach will try to reuse the jump host SSH config for the specified host (login-node) automatically.

It means the user must have login-node SSH config defined.

[github-actions]

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[github-actions]

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