Create 11012.breaking.rst

Author: Dreamsorcerer

CHANGES/11012.breaking.rst

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+Refactored ``ClientRequest`` class. This simplifies a lot of code and improves our type
+checking accuracy. It also better aligns public/private attributes with what we expect
+developers to access safely from a client middleware.
+
+If code subclasses ``ClientRequest``, it is likely that the subclass will need tweaking
+to be compatible with the new version.
+
+-- by :user:`Dreamsorcerer`.