[mlir][ods] ODS ops get an `extraClassDefinition`

Extra definitions are placed in the generated source file for each op class. The substitution `$cppClass` is replaced by the op's C++ class name.

This is useful when declaring but not defining methods in TableGen base classes:

```
class BaseOp<string mnemonic>
    : Op<MyDialect, mnemonic, [DeclareOpInterfaceMethods<SomeInterface>] {
  let extraClassDeclaration = [{
    // ZOp is declared at at the bottom of the file and is incomplete here
    ZOp getParent();
  }];
  let extraClassDefinition = [{
    int $cppClass::someInterfaceMethod() {
      return someUtilityFunction(*this);
    }
    ZOp $cppClass::getParent() {
      return dyn_cast<ZOp>(this->getParentOp());
    }
  }];
}
```

Certain things may prevent defining these functions inline, in the declaration. In this example, `ZOp` in the same dialect is incomplete at the function declaration because ops classes are declared in alphabetical order. Alternatively, functions may be too big to be desired...