feat: add DerpMap::default_from_url

[Frando]

Description

Now that #1143 is merged, the default Derp map can be configured from a single URL for the common case of using DNS and the default STUN port. This PR adds a DerpMap::default_from_url method that just takes a Derp server URL as argument. When writing examples that accept a Derp server from e.g. a CLI argument this makes the Derp map construction simpler.

Change checklist

• Self-review.
• Documentation updates

[rklaehn]

Looks reasonable.

One unrelated thing: I tried out the example and had the server stop once because of a timeout. I think accept_conn producing an error should not terminate the accept loop.

let (peer_id, alpn, conn) = accept_conn(conn).await?;

should be

let Ok((peer_id, alpn, conn)) = accept_conn(conn).await else {
  error!("accept_conn errored");
  continue;
};