Instantiate extension operations from an `OpDef` in python

[aborgna-q]

So we can use the parametric extension operations

op_def: OpDef = hugr.std.collections.EXTENSION.get_op("push")
# Instantiate `push` with some element type
# This method does not exist
concrete_op: ExtOp = op_def.instantiate(args, sig=optional_signature)
builder.add_op(concrete_op, list, elem)

The current workaround requires manually instantiating ops.ExtOp with a signature and arguments, and setting the private _op_def field.