*: change struct evpn_addr to include a union of all evpn route types

*: change struct evpn_addr to include a union of all evpn route types

EVPN prefix depends on the EVPN route type.
Currently, in FRR we have a prefix_evpn/evpn_addr which relates to a evpn prefix.
We need to convert this to encompass an union of various EVPN route-types.

This diff handles the necessary code changes to adopt the new struct evpn_addr.

Signed-off-by: Mitesh Kanjariya <mitesh@cumulusnetworks.com>