Add support for connector factory in connect args

[enocom]

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 #1054.