Add support for gpu-rent.vars and enhance git update functionality

- Updated .gitignore to include gpu-rent.vars.
- Modified env.example to introduce the UPDATE_GIT variable for controlling git updates during execution.
- Implemented Import-GpuRentVars function in gpu-rent.ps1 to load environment variables from gpu-rent.vars.
- Enhanced gpu-rent.sh to support loading variables from gpu-rent.vars and added logic for handling default and extra arguments.
- Updated CLI documentation to reflect the new gpu-rent.vars file and its usage in configuration.
- Improved bootstrap and provisioning logic to conditionally perform git updates based on the new configuration.

tests/test_sshtunnel_shim.py

@@ -0,0 +1,18 @@
+import paramiko
+
+from gpu_rent.tunnel import _patch_paramiko_for_sshtunnel, _ssh_tunnel_forwarder
+
+
+def test_dsskey_shim_allows_sshtunnel_import():
+    # Simulate paramiko>=4 (no DSSKey) then ensure sshtunnel can load.
+    had = getattr(paramiko, "DSSKey", None)
+    if had is not None:
+        delattr(paramiko, "DSSKey")
+    try:
+        _patch_paramiko_for_sshtunnel()
+        assert hasattr(paramiko, "DSSKey")
+        cls = _ssh_tunnel_forwarder()
+        assert cls is not None
+    finally:
+        if had is not None:
+            paramiko.DSSKey = had