feat: Add public Endpoint::new constructor

This can be used to construct new peer endpoints, which is positioned as
the 'default' by naming the method simply `new`.

This roughly follows the logic of the crate private `new` that it
replaces (renamed to `_new`, to be removed in future), with some
differences:

- Errors that occur when establishing the public address or port
  forwarding are returned to the user. As a library, it's been deemed
  preferable that callers handle this and retry, with the relevant
  configuration altered, if desired.

- Configured external IP/port are now combined with the steps to query
  our public address from a peer. This is intended to simplify the logic
  around public address handling – there's now a single sequence of
  steps:

  1. Start with `public_addr = local_addr`.
  2. If we bootstrapped to a peer, get our `visible_addr` from them.
  3. If we configured a specific `external_ip`, override that in
     `public_addr`.
     1. Otherwise, if we have a `visible_addr`, override the IP in
        `public_addr`.
  4. If we configured a specific `external_port`, override that in
     `public_addr`.
     1. Otherwise, if we have a `visible_addr`, override the port in
        `public_addr`.

  Still not exactly "simple", but easier to follow.

- If we bootstrapped to a peer, we always test reachability of our
  determined `public_addr`. There's no reason not to do this – even if
  the endpoint is local-only, it verifies the expectation of the caller
  that the given contacts can reach us (which will work if they are also
  local).