Add AFI/SAFI for Route Target constraints

[grahamjohnston]

Environment

• Nautobot version: >= 1.3.0
• nautobot-bgp-models version: >= 0.9.0

Proposed Functionality

I propose that we add an entry to choices.py that allows for the use of the Route Target constraint AFI/SAFI. This address-family is documented in RFC4684 and is useful in providing automatic filtering of VPN routes advertisements based on an advertised list of route-target values from a peer.

This feature request makes no changes to any logic or "code" in the plugin, it only adds a new choice to the fixed list of AFI/SAFI that are available for use.  

Use Case

By using this address-family, I can easily limit VPN routes that I receive on one peer by advertising a list of route-targets to a remote peer.

[mzbroch]

We have been following https://github.com/openconfig/public/blob/master/release/models/bgp/openconfig-bgp-types.yang types in implementation.

For our understanding, can You enclose config snippets to represent Your configuration?