[mlir:PDL] Adjust the assembly format for AttributeOp to avoid conflicts with DictionaryAttr

Switch the attribute creation operations to use attr-dict-with-
keyword to avoid conflicts (in the case of pdl.attribute) and
confusion(in the case of pdl_interp.create_attribute) with
having a DictionaryAttr as a value and specifying the
attributes of the operation itself (as a dictionary).

Differential Revision: https://reviews.llvm.org/D114815