Support dynamic transient workspace size queries

Fusilli currently assumes transient workspace size can be read as a constant from IREE module metadata. Dynamic-shape graphs that need runtime-dependent workspace sizes expose an iree.abi.transients.size function instead, and Graph::getWorkspaceSize() reports NOT_IMPLEMENTED.

This blocks running dynamic-shape multiple-dispatch cases such as dynamic batch MIN/MAX reductions. Add support for invoking the size function with runtime variant-pack shapes, then allocate and pass the resulting workspace.