This commit adds support for providing a connector factory, a callable
that provides full control over how the network connection to the
PostgreSQL server is established. When specified, this factory replaces
the default connection logic. The callable receives the following
arguments:

- *proto_factory* - a callable that returns the asyncpg protocol
  instance
- *host* - the target hostname (positional)
- *port* - the target port (positional)
- *loop* - the event loop (keyword argument)
- *ssl* - the SSL context, or ``None`` (keyword argument)

The callable must return an awaitable that resolves to a
`(transport, protocol)` tuple, compatible with
`asyncio.loop.create_connection`.

This is useful for scenarios such as connecting through a proxy,
establishing an SSH tunnel, or performing custom socket setup
before the PostgreSQL protocol begins.

Usage looks like this:

```
async def my_connector(proto_factory, host, port, *, loop, ssl):
    tunnel_sock = await open_ssh_tunnel(host, port)
    return await loop.create_connection(
        proto_factory, sock=tunnel_sock, ssl=ssl)

conn = await asyncpg.connect(
    connector_factory=my_connector,
    host='db.example.com',
    user='postgres',
)
```

Fixes MagicStack#1054.