util: add new util.experimentalflags.get_{bool,string} helpers

This commit adds two new helpers:
- util.experimentalflags.get_bool()
- util.experimentalflags.get_string()
similar to what we added in the images library in PR:
https://github.com/osbuild/images/pull/1248

The idea is that we provide experimentalflags for osbuild via
an environment like `OSBUILD_EXPERIMENTAL` and for those we
make no API promises. This will be initially used for better
debug of qemu-user.

osbuild/util/experimentalflags.py

@@ -0,0 +1,31 @@
+"""Handling of experimental environment flags"""
+
+import os
+from typing import Any, Dict
+
+
+def _experimental_env_map() -> Dict[str, Any]:
+    env_map: Dict[str, Any] = {}
+    for exp_opt in os.environ.get("OSBUILD_EXPERIMENTAL", "").split(","):
+        l = exp_opt.split("=", maxsplit=1)
+        if len(l) == 1:
+            env_map[exp_opt] = "true"
+        elif len(l) == 2:
+            env_map[l[0]] = l[1]
+    return env_map
+
+
+def get_bool(option: str) -> bool:
+    env_map = _experimental_env_map()
+    opt = env_map.get(option, "")
+    # sadly python as no strconv.ParseBool() like golang so we roll our own
+    if opt.upper() in {"1", "T", "TRUE"}:
+        return True
+    if opt.upper() in {"", "0", "F", "FALSE"}:
+        return False
+    raise RuntimeError(f"unsupport bool val {opt}")
+
+
+def get_string(option: str) -> str:
+    env_map = _experimental_env_map()
+    return str(env_map.get(option, ""))