all: add extra information to serialized endpoints

magicsock.Conn.ParseEndpoint requires a peer's public key,
disco key, and legacy ip/ports in order to do its job.
We currently accomplish that by:

* adding the public key in our wireguard-go fork
* encoding the disco key as magic hostname
* using a bespoke comma-separated encoding

It's a bit messy.

Instead, switch to something simpler: use a json-encoded struct
containing exactly the information we need, in the form we use it.

Our wireguard-go fork still adds the public key to the
address when it passes it to ParseEndpoint, but now the code
compensating for that is just a couple of simple, well-commented lines.
Once this commit is in, we can remove that part of the fork
and remove the compensating code.

Signed-off-by: Josh Bleecher Snyder <josharian@gmail.com>

wgengine/userspace_test.go

@@ -104,7 +104,7 @@ func TestUserspaceEngineReconfig(t *testing.T) {
 					AllowedIPs: []netaddr.IPPrefix{
 						{IP: netaddr.IPv4(100, 100, 99, 1), Bits: 32},
 					},
-					Endpoints: discoHex + ".disco.tailscale:12345",
+					Endpoints: wgcfg.Endpoints{DiscoKey: dkFromHex(discoHex)},
 				},
 			},
 		}