Support custom converters for user keyword argument type hints

Context

User keyword [Arguments] type hints (${name: type}, added in RF 7.0) currently only accept Python built-ins and a small standard-library set. Per the User Guide, "Variable type conversion":

"The biggest limitations compared to the argument conversion with library keywords is that Enum and TypedDict conversions are not supported and that custom converters cannot be used. These limitations may be lifted in the future versions."

In practice, a hint like ${point: Point} (a custom class with a registered converter, e.g. Point.from_string) fails at parse time with Unrecognized type 'Point', even when Point is registered as a converter on an imported library. Library keywords get full custom-type auto-conversion (#4088); user keywords cannot use the same converters at all today.

Request

Allow user keyword type hints to resolve custom types the same way library keyword arguments do, either via converters registered on an imported library, or via whatever mechanism comes out of #4944 (global scope type converters). This would make user keywords and library keywords behave consistently for the same custom type, closing a gap that currently forces a manual workaround anywhere a user keyword needs a typed argument.

Related

• #4088, "Ability to register custom converters for keyword arguments" (closed, implemented): the library-keyword side of custom converters that this issue would extend to user keywords.
• #4944, "Support global scope type converters" (open): a global converter registry would likely be the natural mechanism to also unlock this for user keywords, but that issue doesn't explicitly call out user keyword type hints as a target, hence this separate, more specific request.