Prop 971: Add MsgExec to Allowed ICA Messages List

This proposal adds MsgExec to the allowed ICA messages list, extending existing authz execution support from EOAs and contracts to ICAs. It aligns Osmosis with other chains (Cosmos Hub, Neutron, Injective, Elys) where MsgExec is already supported for ICAs. Forum Post: https://forum.osmosis.zone/t/add-msgexec-to-allowed-ica-messages-list