[mlir][transform] Add transform.get_operand op (#78397)

Similar to `transform.get_result`, except it returns a handle to the
operand indicated by a positional specification, same as is defined for
the linalg match ops.

Additionally updates `get_result` to take the same positional specification.
This makes the use case of wanting to get all of the results of an
operation easier by no longer requiring the user to reconstruct the list
of results one-by-one.