gh-76785: Add PyInterpreterConfig Helpers (gh-117170)

These helpers make it easier to customize and inspect the config used to initialize interpreters.  This is especially valuable in our tests.  I found inspiration from the PyConfig API for the PyInterpreterConfig dict conversion stuff.  As part of this PR I've also added a bunch of tests.

Python/config_common.h

@@ -0,0 +1,36 @@
+
+static inline int
+_config_dict_get(PyObject *dict, const char *name, PyObject **p_item)
+{
+    PyObject *item;
+    if (PyDict_GetItemStringRef(dict, name, &item) < 0) {
+        return -1;
+    }
+    if (item == NULL) {
+        // We do not set an exception.
+        return -1;
+    }
+    *p_item = item;
+    return 0;
+}
+
+
+static PyObject*
+config_dict_get(PyObject *dict, const char *name)
+{
+    PyObject *item;
+    if (_config_dict_get(dict, name, &item) < 0) {
+        if (!PyErr_Occurred()) {
+            PyErr_Format(PyExc_ValueError, "missing config key: %s", name);
+        }
+        return NULL;
+    }
+    return item;
+}
+
+
+static void
+config_dict_invalid_type(const char *name)
+{
+    PyErr_Format(PyExc_TypeError, "invalid config type: %s", name);
+}