[mlir] Expand operand adapter to take attributes

* Enables using with more variadic sized operands;
* Generate convenience accessors for attributes;
  - The accessor are named the same as their name in ODS and returns attribute
    type (not convenience type) and no derived attributes.

This is first step to changing adapter to support verifying argument
constraints before the op is even created. This does not change the name of
adaptor nor does it require it except for ops with variadic operands to keep this change smaller.

Considered creating separate adapter but decided against that given operands also require attributes in general (and definitely for verification of operands and attributes).

Differential Revision: https://reviews.llvm.org/D80420