[mlir][Python] Add python binding to create DenseElementsAttribute.

* Interops with Python buffers/numpy arrays to create.
* Also cleans up 'get' factory methods on some types to be consistent.
* Adds mlirAttributeGetType() to C-API to facilitate error handling and other uses.
* Punts on a lot of features of the ElementsAttribute hierarchy for now.
* Does not yet support bool or string attributes.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D89363