In the risserver every bmp server is identified by its address,
specified in the config, but the GetRouters() grpc only presents us with
the name acquired via the BMP session. To clarify the field usage the
router field is renamed to sys_name and an address field is added. The
address field is of type string to match the other grpc methods, which
require a router address to be specified as string.

Co-authored-by: takt <oliver.herms@exaring.de>

* Implement a GetRouters RPC to RIS

This RPC returns all configured routers and their VRF IDs. This can be
used by RIS clients to discovery the routers. The router and the VRF ID
is a required parameter for all other RPCs.

* Update grpc to v1.28.0 to support protoc-gen-go v1.21.0

* Fix test

Co-authored-by: Oliver Herms <oliver.peter.herms@gmail.com>

* Make BMP server VRF aware

* Implement RIS service

* Remove binary

* Fix .gitignore

* Implement more RPCs

* Implement RIB Observer

* Fix tests

* Implement RIB Dump

* Add support for translating proto BGP Paths into internal paths

* Cleanup

* Fix peer down

* Implement buffered update receiver. Fix route propagation.

* Add metrics

* Add VRF metrics

* Add router label to VRF metrics

* Add peer metrics

* Fix tests

* Fix test

* Fix test

* Fix build

* Add tests

* Add test for unknown path attributes

* Add tests

* Extend test

* Standardize grpc API

* Fix tests