Add support for typing.Literal in Mapped

Add support for typing.Literal in Mapped

Added support for :pep:`586` ``Literal`` to be used in the
:paramref:`_orm.registry.type_annotation_map` as well as within
:class:`.Mapped` constructs. To use custom types such as these, they must
appear explicitly within the :paramref:`_orm.registry.type_annotation_map`
to be mapped.  Pull request courtesy Frederik Aalund.

As part of this change, the support for :class:`.sqltypes.Enum` in the
:paramref:`_orm.registry.type_annotation_map` has been expanded to include
support for ``Literal[]`` types consisting of string values to be used,
in addition to ``enum.Enum`` datatypes.    If a ``Literal[]`` datatype
is used within ``Mapped[]`` that is not linked in
:paramref:`_orm.registry.type_annotation_map` to a specific datatype,
a :class:`.sqltypes.Enum` will be used by default.

Fixed issue involving the use of :class:`.sqltypes.Enum` within the
:paramref:`_orm.registry.type_annotation_map` where the
:paramref:`_sqltypes.Enum.native_enum` parameter would not be correctly
copied to the mapped column datatype, if it were overridden
as stated in the documentation to set this parameter to False.

Fixes: #9187
Fixes: #9200
Closes: #9191
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9191
Pull-request-sha: 7d13f705307bf62560fc831f6f049a425d411374

Change-Id: Ife3ba2655f4897f806d6a9cf0041c69fd4f39e9d