Change Optional to Ternary in dash_acl.p4

https://github.com/sonic-net/DASH/blob/e39249bf647aba16736991364052706bea3176c8/dash-pipeline/bmv2/dash_acl.p4#L22

Per our Community discussion today, could we change '**optional**' to '**ternary**' ?

Opening Issue per @marian-pritsak , @mgheorghe , and @r12f 