Derive RoutingProtocol.description from device, VRF and protocol instead of requiring it #86

Context. RoutingProtocol is the generic every routing protocol extension inherits from.
Its description is a mandatory, globally unique Text attribute, so every BGP session,
OSPF process, PIM instance and aggregate route in the library forces the operator to hand
type a string that no two objects may share. The obvious value is mechanical: device plus
VRF plus protocol.

Current marker.

extensions/routing/routing.yml:18
      # TODO: Should be computed later

sitting above:

      - name: description
        kind: Text
        unique: true
        optional: false
        description: "Description of the protocol"
        order_weight: 1100

What this asks for. Convert description to a Jinja2 computed attribute derived from the
protocol's own device, VRF and protocol type, or replace it with a proper identity so the
mandatory-unique free-text field goes away. Not a v2.0 edit: a computed attribute must be
optional: true and read_only: true, which interacts with the HFID and uniqueness work
the review already does under CC-4, and the derivation has to hold for all five protocol
extensions plus routing_bgp_rr and peering_ixp.

Origin: pass-1 review of #75, packet RTG-CC1, CC-8 bucket (c).