gh-115999: Specialize `STORE_ATTR` in free-threaded builds. (gh-127838)

* Add `_PyDictKeys_StringLookupSplit` which does locking on dict keys and
  use in place of `_PyDictKeys_StringLookup`.

* Change `_PyObject_TryGetInstanceAttribute` to use that function
  in the case of split keys.

* Add `unicodekeys_lookup_split` helper which allows code sharing
  between `_Py_dict_lookup` and `_PyDictKeys_StringLookupSplit`.

* Fix locking for `STORE_ATTR_INSTANCE_VALUE`.  Create
  `_GUARD_TYPE_VERSION_AND_LOCK` uop so that object stays locked and
  `tp_version_tag` cannot change.

* Pass `tp_version_tag` to `specialize_dict_access()`, ensuring
  the version we store on the cache is the correct one (in case of
  it changing during the specalize analysis).

* Split `analyze_descriptor` into `analyze_descriptor_load` and
  `analyze_descriptor_store` since those don't share much logic.
  Add `descriptor_is_class` helper function.

* In `specialize_dict_access`, double check `_PyObject_GetManagedDict()`
  in case we race and dict was materialized before the lock.

* Avoid borrowed references in `_Py_Specialize_StoreAttr()`.

* Use `specialize()` and `unspecialize()` helpers.

* Add unit tests to ensure specializing happens as expected in FT builds.

* Add unit tests to attempt to trigger data races (useful for running under TSAN).

* Add `has_split_table` function to `_testinternalcapi`.