[mlir][CAPI] Attribute set/remove on operations.

* New functions: mlirOperationSetAttributeByName, mlirOperationRemoveAttributeByName
* Also adds some *IsNull checks and standardizes the rest to use "static inline" form, which makes them all non-opaque and not part of the ABI (which is desirable).
* Changes needed to resolve TODOs in npcomp PyTorch capture.

Differential Revision: https://reviews.llvm.org/D88946