[mlir][irdl] Add `irdl.attributes` operation for defining named attributes

This commit introduces the `irdl.attributes` operation, which allows defining named attributes for the parent operation. Each attribute is defined with a name and a type constraint.

Example usage:
```
irdl.dialect @example {
  irdl.operation @attr_op {
    %0 = irdl.any
    %1 = irdl.is i64
    irdl.attributes {
      "attr1" = %0,
      "attr2" = %1
    }
  }
}
```
In this example the operation will expect an arbitrary attribute "attr1"
and an attribute "attr2" with value `i64`.

Reviewed By: math-fehr, Mogball

Differential Revision: https://reviews.llvm.org/D152618