[mlir][python] provide access to function argument/result attributes

Without this change, these attributes can only be accessed through the generic
operation attribute dictionary provided the caller knows the special operation
attribute names used for this purpose. Add some Python wrapping to support this
use case.

Also provide access to function arguments usable inside the function along with
a couple of quality-of-life improvements in using block arguments (function
arguments being the arguments of its entry block).

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D110758