Introduce an internal TypeGuard library, globus_sdk._guards

[sirosen]

This is a very modest internal improvement, but it does lead to cleaner and clearer internal code which is slightly stricter in certain edge-cases.

Some of the improvements are more obvious than others. For example, here's a usage site which reads more easily now:

-        if isinstance(self._dict_data.get("errors"), list) and all(
-            isinstance(subdoc, dict) for subdoc in self._dict_data["errors"]
-        ):
+        if _guards.is_list_of(self._dict_data.get("errors"), dict):

Is this shorter / leaner / smaller? Not really.
Limiting ourselves to the src/ tree, _guards.py itself is 62 LOC, and we have +110,-68.
So if we discount the _guards.py module itself, it's marginally shorter. That's not really the point though.

This is more of a marginal improvement around the edges to how we do things, especially checking of list contents.
For example, in the TimerAPIError class, we have these changes:

-        elif isinstance(self._dict_data.get("detail"), list):
+        elif _guards.is_list_of(self._dict_data.get("detail"), dict):
 ... # context ...
-            self.errors = [
-                ErrorSubdocument(d)
-                for d in self._dict_data["detail"]
-                if isinstance(d, dict)
-            ]
+            self.errors = [ErrorSubdocument(d) for d in self._dict_data["detail"]]

These are not quite the same.
Because the only tool ready-to-hand before was isinstance(X, list), that was used, and then we had to handle, later, the contents of that list. Putting _guards.is_list_of(X, dict) in easy reach eliminates that potential point of divergence between various components.

In the first 2 commits, I went after the "easy" version of this. It works nicely.
In the second 2 commits, I went after an aspirational goal to make the GARE validators part of a broader suite of behaviors we have within the SDK. They add a bit of complexity (reduce / _Reducer), but I think the result is nice.

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📚 Documentation preview 📚: https://globus-sdk-python--798.org.readthedocs.build/en/798/

[sirosen]

I've reverted and removed the fancy _Reducer. Per discussion, although it's cool, it's not intuitive enough to offset the cost of having to make sure that everyone understands it, or the potential maintenance burden if mypy or other type checkers start to treat it differently.