Adding Type and Nullability Validation to exposed fields

Hello 👋

I am creating this ticket to ask if it would be possible to introduce type validation to exposed fields.

Currently there is not checking if the exposed fields are of a specified type. We can add documentation to specify a type (that is then exposed as part of the swagger docs) but if there is a mismatch there is no way for the server to validate this break in contract.

I am thinking we can add type validation (using dry-types or dry-validation) to the exposed fields (maybe after formatting). This would of course be opt-in with the expose DSL method accepting two new options

type (this is the type the field should be or be coercible to) a dry type or standard ruby type like Numeric.
nullable a boolean field to validate if the value can be nil or not.
These options then can be used to populate the swagger documentation automatically without specifying them in the documents hash.

I am happy to start contribute this feature but first want to check if the maintainers @dblock & @LeFnord are okay with it 😅

No files or tests are named. Start by reviewing the expose DSL and the existing documents hash handling, then determine how type and nullable options would affect exposed values and generated Swagger documentation; done means the maintainers agree on the design and the behavior is covered by tests.