Proto: migrate AsyncFuncExec #23514

Part of #23494.

async_func.rs. Single child + N async-function exprs (name + func expr); all via the ctx, no codec.

Follow the pattern in #23494 (reference impl: #23495 / ProjectionExec; full end-state draft: #23496). Add try_to_proto inside impl ExecutionPlan for X, add the inherent X::try_from_proto, repoint the decode arm, and delete the old central arm in the same PR (that is the proof the hook is reached). Keep the wire format byte-for-byte identical.