code-gen(experimental/crud): support custom readable type

[dirkdev98]

Support a custom readable type on T.crud().fields({ readable: T.reference("my", "type") }). This has the following characteristics:

• The type is used instead of the generated generated xxxItem in list, single and create routes
• No changes are necessary on the generated query builders, since they can be mutated already in the 'preModifier' functions, to add things like extra joins
• The xxxTransfrom will be skipped and instead an transform function will be accepted when registering the CRUD routes. Each route result will pass through that transform for list, single and create routes.