util: add helper to parse size strings

Code is based on `common.DataSizeToUint64` in Composer, with a
modification to allow `unlimited` so that the result is compatible
with `fscache.MaximumSizeType`.

[1] f4aed3e6e2/internal/common/helpers.go (L46)

test/mod/test_util_parsing.py

@@ -0,0 +1,39 @@
+"""Unit tests for osbuild.util.parsing"""
+
+import pytest
+
+from osbuild.util import parsing
+
+
+def test_parse_size():
+    cases = [
+        ("123", True, 123),
+        ("123 kB", True, 123000),
+        ("123 KiB", True, 123 * 1024),
+        ("123 MB", True, 123 * 1000 * 1000),
+        ("123 MiB", True, 123 * 1024 * 1024),
+        ("123 GB", True, 123 * 1000 * 1000 * 1000),
+        ("123 GiB", True, 123 * 1024 * 1024 * 1024),
+        ("123 TB", True, 123 * 1000 * 1000 * 1000 * 1000),
+        ("123 TiB", True, 123 * 1024 * 1024 * 1024 * 1024),
+        ("123kB", True, 123000),
+        ("123KiB", True, 123 * 1024),
+        (" 123", True, 123),
+        ("  123kB", True, 123000),
+        ("  123KiB", True, 123 * 1024),
+        ("unlimited", True, "unlimited"),
+        ("string", False, 0),
+        ("123 KB", False, 0),
+        ("123 mb", False, 0),
+        ("123 PB", False, 0),
+        ("123 PiB", False, 0),
+
+    ]
+
+    for s, success, num in cases:
+        if not success:
+            with pytest.raises(TypeError):
+                parsing.parse_size(s)
+        else:
+            res = parsing.parse_size(s)
+            assert res == num, f"{s} parsed as {res} (wanted {num})"