allow TypeEngine classes to export their repr() parameters

allow TypeEngine classes to export their repr() parameters

Added a new :class:`.GenericRepr` class and :meth:`.TypeEngine.repr_struct`
method to provide better control over type representation. The
:class:`.TypeDecorator` class now properly displays its decorated type's
parameters in its ``__repr__()``, allowing introspection tools and
libraries like Alembic to better understand the structure of decorated
types, in particular for complex "schema" types such as :class:`.Enum` and
:class:`.Boolean`. Type classes can override
:meth:`.TypeEngine.repr_struct` to customize their representation
structure, and the returned :class:`.GenericRepr` object allows for
modifications such as changing the displayed class name.

Fixes: #13140
Change-Id: Ie41d249cfea56686b16c895b74ae03721207170b