[mlir][Interfaces] Add a extraSharedClassDeclaration field

This field allows for defining a code block that is placed in both the interface
and trait declarations. This is very useful when defining a set of utilities to
expose on both the Interface class and the derived attribute/operation/type.

In non-static methods, `$_attr`/`$_op`/`$_type` (depending on the type of
interface) may be used to refer to an instance of the IR entity. In the interface
declaration, this is an instance of the interface class. In the trait declaration,
this is an instance of the concrete entity class (e.g. `IntegerAttr`, `FuncOp`, etc.).

Differential Revision: https://reviews.llvm.org/D116961