EVPN-VXLAN: Preserve 802.1p CoS priority

Enables devices to preserve and copy IEEE 802.1p priority bits (P bits) between customer VLAN tags (C tags) and service VLAN tags (S tags) during EVPN VXLAN encapsulation and decapsulation.

When enabled, the C tag P bit is copied to the S tag during VLAN push operations, and preserved during VLAN pop operations, ensuring consistent traffic prioritization across EVPN VXLAN networks. Turn on this behavior using the vxlan-enable-dot1p-copy option at the [edit forwarding-options] hierarchy level. This option applies only to SP style EVPN VXLAN configurations.