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_varsfile.py

@@ -0,0 +1,32 @@
+from pathlib import Path
+
+from gpu_rent.varsfile import apply_vars_file, parse_vars_file, split_args
+
+
+def test_parse_vars_file(tmp_path: Path):
+    path = tmp_path / "gpu-rent.vars"
+    path.write_text(
+        "# comment\n"
+        "GPU_RENT_DEFAULT_ARGS=up --yes\n"
+        "export UPDATE_GIT=false\n"
+        "QUOTED=\"a b\"\n",
+        encoding="utf-8",
+    )
+    data = parse_vars_file(path)
+    assert data["GPU_RENT_DEFAULT_ARGS"] == "up --yes"
+    assert data["UPDATE_GIT"] == "false"
+    assert data["QUOTED"] == "a b"
+
+
+def test_apply_does_not_override(tmp_path: Path, monkeypatch):
+    path = tmp_path / "gpu-rent.vars"
+    path.write_text("FOO=from-file\n", encoding="utf-8")
+    monkeypatch.setenv("FOO", "from-env")
+    apply_vars_file(path, override=False)
+    import os
+
+    assert os.environ["FOO"] == "from-env"
+
+
+def test_split_args():
+    assert split_args("up --yes --no-update") == ["up", "--yes", "--no-update"]