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.

test/mod/test_util_experimentalflags.py

@@ -0,0 +1,39 @@
+#
+# Test for the util.experimentalflags
+#
+import pytest
+
+from osbuild.util import experimentalflags
+
+
+@pytest.mark.parametrize("env,expected_foo", [
+    # implicit false
+    ("", False),
+    ("bar", False),
+    # explicit true
+    ("foo", True),
+    ("foo,bar", True),
+    ("foo=1", True),
+    ("foo=1,bar", True),
+    ("foo=true", True),
+    ("foo=t", True),
+    # explicit falgs
+    ("foo=false", False),
+    ("foo=0", False),
+    ("foo=f", False),
+    ("foo=F", False),
+    ("foo=FALSE", False),
+])
+def test_experimentalflags_bool(monkeypatch, env, expected_foo):
+    monkeypatch.setenv("OSBUILD_EXPERIMENTAL", env)
+    assert experimentalflags.get_bool("foo") == expected_foo
+
+
+@pytest.mark.parametrize("env,expected_key", [
+    ("", ""),
+    ("key=val", "val"),
+    ("foo,key=val,bar", "val"),
+])
+def test_experimentalflags_string(monkeypatch, env, expected_key):
+    monkeypatch.setenv("OSBUILD_EXPERIMENTAL", env)
+    assert experimentalflags.get_string("key") == expected_key