proposal: expose transport stats

Nobody has claimed this yet.

Motivating issue: Get accurate RTT measurement for yamux (https://github.com/libp2p/go-yamux/pull/70).

We could introduce an `transport.StatTransport` (suggestions for a better name appreciated!) interface that transports can (but don't need to) implement:
```go
type StatTransport {
Stat() Stat
}

type Stat struct {
RTT time.Duration
RTTVariance time.Duration
}
```

Both TCP and QUIC should be able to expose these values easily. We already have an implementation for TCP: https://github.com/libp2p/go-libp2p/blob/fcf408c65d99f9a2f45c761a1003d66d3a664f30/p2p/transport/tcp/metrics.go#L240-L249
quic-go could expose these on the `quic.Connection`, the congestion controller is already keeping track of these values.

Open question: In the future, we'll probably want to add more values to `Stat`. Obvious candidates are congestion window, packet loss rate, counters for bytes sent and received, etc. Will every transport be able to determine these values?

cc @vyzo @Stebalien @MarcoPolo @BigLep

Start with the proposed transport.StatTransport and Stat interfaces in the issue, then read p2p/transport/tcp/metrics.go around the referenced TCP implementation. Check how quic.Connection exposes congestion-controller values. Done would require agreement on the interface, supported statistics, and transport behavior, which the issue leaves open.

Written by the indexing model from the issue text.