generalize cache_ok to UserDefinedType

Extended the ``cache_ok`` flag and corresponding warning message if this
flag is not defined, a behavior first established for
:class:`.TypeDecorator` as part of :ticket:`6436`, to also take place for
:class:`.UserDefinedType`, by generalizing the flag and associated caching
logic to a new common base for these two types, :class:`.ExternalType`.

The change means any current :class:`.UserDefinedType` will now cause SQL
statement caching to no longer take place for statements which make use of
the datatype, along with a warning being emitted, unless the class defines
the :attr:`.UserDefinedType.cache_ok` flag as True. If the datatype cannot
form a deterministic, hashable cache key derived from its arguments, it may return
False which will continue to keep caching disabled but will suppress the
warning. In particular, custom datatypes currently used in packages such as
SQLAlchemy-utils will need to implement this flag. The issue was observed
as a result of a SQLAlchemy-utils datatype that is not currently cacheable.

Fixes: #7319
Change-Id: Ie0b5d4587df87bfe66d2fe7cd4585c3882584575

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