EDSC bindings: expose generic Op construction interface

EDSC Expressions can now be used to build arbitrary MLIR operations identified
by their canonical name, i.e. the name obtained from
`OpClass::getOperationName()` for registered operations.  Expose this
functionality to the C API and Python bindings.  This exposes builder-level
interface to Python and avoids the need for experimental Python code to
implement EDSC free function calls for constructing each op type.

This modification required exposing mlir::Attribute to the C API and Python
bindings, which only supports integer attributes for now.

This is step 4/n to making EDSCs more generalizable.

PiperOrigin-RevId: 236306776