Port forwarding (`multipass forward`)

Forwarding instance ports out onto multipassd's listening address(es):

```
# can pass multiple --forward rules
$ multipass launch --forward [:]
Port forwarding configured on:
Port : :

$ multipass forward [:]
Port forwarding configured on: :

$ multipass unforward [:] [[:] ...]

$ multipass info
[...]
Port forwarding:

```

Where it's possible / makes sense, use hypervisor for this, otherwise have multipassd listen.

Start by tracing the `multipass launch --forward`, `multipass forward`, `multipass unforward`, and `multipass info` entry points and the hypervisor networking integration. Define how multiple rules, default listening addresses, and removal should behave, then verify that configured forwarding is reported as shown.

Written by the indexing model from the issue text.