This is a preparation for the upcoming new BGPv2-native CLI and metrics.

In BGPv2, the peer is identified by the explicit name instead of IP
address. The reason for this spec is that makes it easier for users to
identify dynamically discovered peers on CLI or metrics.

However, the CLI and metrics output are based on the information
retrieved from the RouterManager which doesn't have a peer name => peer
IP address correspondence. Having this mapping in the RouterManager is
tricky because the peer discovery is done by the reconcilers.

The solution here is keeping the peer name in the GoBGP's Peer object.
While GoBGP still identifies peer with IP address, we have a Description
field that accepts arbitrary string. We can keep the peer name there.

For the sake of the future extensibility, the name is encoded as a JSON
string. Whenever we want to encode additional metadata, we can introduce
a new JSON field.

The agent API field is extended with name:

```
$ cilium-dbg bgp peer -o json
[
  {
    "configured-hold-time-seconds": 90,
    "configured-keep-alive-time-seconds": 30,
    "connect-retry-time-seconds": 120,
    "ebgp-multihop-ttl": 1,
    "families": [
      {
        "afi": "ipv6",
        "safi": "unicast"
      },
      {
        "afi": "ipv4",
        "safi": "unicast"
      }
    ],
    ...
    "name": "peer0",
    "peer-address": "10.0.0.1",
    "peer-port": 179,
    "remote-capabilities": [],
    "session-state": "active"
  }
]
```

Note that cilium-cli is not yet updated with this change because we will
introduce a completely new CLI output shortly, so it doesn't make sense
to augment the current output with name.

The TestGetPeerStatus test case used to identify peer with IP + ASN.
Change it to leverage this API.

Signed-off-by: Yutaro Hayakawa <yutaro.hayakawa@isovalent.com>