Add EVPN support to BGP spine-leaf playbook

[eduolivares]

Add EVPN-specific configuration to the existing BGP spine-leaf setup, gated by an enable_evpn variable:

• Leaf FRR: skip BGP password when EVPN is enabled (the EVPN extension does not configure one), add advertise-all-vni
• Router FRR: add VRF with VNI and BGP VRF instance for EVPN Type-5 route import/export
• Router play: create VXLAN interface, bridge, and VRF device so the router can reach VMs on the EVPN overlay

Configurable variables: enable_evpn (bool), evpn_vni (int, default 1), evpn_vxlan_port (int, default 4789).

Assisted-By: Claude Opus 4.6

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