API additions for [1].
* Added a new router attribute: external_gateways;
* Added new API definitions for:
PUT add_external_gateways
PUT update_external_gateways
PUT remove_external_gateways
* Added extensions for each of the new router-level attributes:
* enable_default_route_ecmp
* enable_default_route_bfd
* Combined the validation logic for the external_gateway_info type
across extensions (l3_ext_gw_mode, qos_gateway_ip and the new
extension called l3_ext_gw_multihoming).
[1] https://review.opendev.org/c/openstack/neutron-specs/+/870030/
Change-Id: I2618475636b2bb9bfd743a62f5d4859d4f68a547
Related-Bug: #2002687
e52a9372f7