net/netip: add missing encoding.BinaryUnmarshaler to AddrPort and Prefix

The Addr type got an encoding.BinaryUnmarshaler implementation, but not
AddrPort and Prefix. This commit adds the missing implementation of that
interface to these types. It also adds two round trip tests that follow
the template of the existing one for Addr.

Updates #49298.

Change-Id: Iac633aed8aac579960815bb64d06ff3181214841
Reviewed-on: https://go-review.googlesource.com/c/go/+/360875


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>