auto-relay: automatically make reservations on relays as we discover them

Description

Today, a user of rust-libp2p can make a reservation on a relay by instructing the Swarm to listen_on a relayed address, i.e. one with /p2p-circuit in it.

We should develop a module similar to go-libp2ps AutoRelay that does this automatically as we connect to relays:

• Upon a new connection, check the supported protocols on that connection for the relay "hop" protocol.
• Based on some configuration (to be decided), conclude that we should make a new reservation.
• Emit a ToSwarm::ListenOn command with a constructed /p2p-circuit address for the discovered relay.

Motivation

Give users a module they can drop into their Behaviour that automatically creates reservations as we connect to relays.

Requirements

1. The number of reservations we make must be bounded by something, i.e. maximum 10. Should probably be configurable.
2. We may want to make this conditional on our number of external addresses. If we don't have any external addresses, reservations will be useful because it means we are likely behind a NAT.

Open questions

No response

Are you planning to do it yourself in a pull request ?

No

Start by reading the existing Swarm listen_on flow, relay reservation handling, and Behaviour/ToSwarm interface, then compare them with go-libp2p's AutoRelay. Define the reservation policy and configuration, including the maximum count and external-address condition; done means discovered relay connections can trigger bounded, constructed /p2p-circuit ListenOn commands.