[mlir] Make `extraClassOf` compile with attribute and type interfaces (#66292)

Using `extraClassOf` currently does not work with attribute or type
interfaces as the generated code calls `getInterfaceFor`, a private
method of `AttributeInterface` and `TypeInterface` respectively.

This PR fixes this by applying the same change that has been done to
`OpInterface` in the past: Make `getInterfaceFor` a protected member of
the class, allowing the generated code in subclasses to use it.

An attribute and type interface with `extraClassOf` have been added as
interfaces in the test dialect to ensure it compiles without errors.