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_clone_ext_update.py

@@ -0,0 +1,15 @@
+from gpu_rent.remote.clone_ext import do_update, is_sha
+
+
+def test_is_sha():
+    assert is_sha("a" * 40)
+    assert not is_sha("main")
+
+
+def test_do_update_reads_flag(tmp_path, monkeypatch):
+    flag = tmp_path / "gpu-rent-update-git"
+    monkeypatch.setattr("gpu_rent.remote.clone_ext.UPDATE_PATH", flag)
+    flag.write_text("0\n", encoding="utf-8")
+    assert do_update() is False
+    flag.write_text("1\n", encoding="utf-8")
+    assert do_update() is True