Add "technique" attribute to the bgpvpn-connection creation API

The backend which implements the bgpvpn API should report back to the admin what kind of "technique" it supports; so that if several techniques are available for a connection type (l3 or l2), the admin might specify one of them.

This is complementary to the type field that can be seen by the user.

Already known "techniques" are :
for type l3 : ipvpn (RFC4364) and evpn-prefix (draft-ietf-bess-evpn-prefix-advertisement)
for type l2 : evpn (RFC7432)

APIs to retrieve those techniques are part of the spec :
https://review.openstack.org/#/c/177740