topology: simplify local address configuration

[matzf]

The router interface section of the topology.json file previously had two fields to specify the local address of the router:

• public: IP:port
• bind: IP

The idea was that normally only public is supplied. When behind NAT, then bind could override the IP address on which the router binds locally, and public would be the address that is visible to the remote router. The only place where this could be relevant is the router BFD keep-alive messages, as these contain the local address of the router in the SCION address header. There is, however, no check that enforces that this matches the underlay address.
In practice, if bind is supplied, the IP of public has simply been ignored.

This change replaces public and bind with a new local field. The old fields are deprecated, but still processed for backwards compatibility.
The local support omitting the IP to bind on any local IP. This was previously possible only by explicitly configuring a zero IP address.

Aside:

• change the internal representation from net.UDPAddr to netip.AddrPort. Note that this changes the representation in the json dump of the /topology status page.
• remove unused BRNames

[matzf]

This change is 

[oncilla]

Reviewed 36 of 36 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @matzf)