[mlir][python] Usability improvements for Python bindings

Provide a couple of quality-of-life usability improvements for Python bindings,
in particular:

  * give access to the list of types for the list of op results or block
    arguments, similarly to ValueRange->TypeRange,

  * allow for constructing empty dictionary arrays,

  * support construction of array attributes by concatenating an existing
    attribute with a Python list of attributes.

All these are required for the upcoming customization of builtin and standard
ops.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D110946