[1.7.0] Custom resolver option

[technige]

This PR adds an option to pass a custom resolver function into the driver configuration. This allows an override of the regular getaddrinfo call used for DNS resolution, introducing the possibility for multiple addresses to be provided for bootstrapping the routing process.

[lutovich]

Maybe we can say that it is a hostname or IP address resolver and default implementation performs DNS resolution.

[technige]

👍 Yes, that's better wording.

[eastlondoner]

I'm not sure this should replace getaddrinfo.

I'd rather that it just comes before getaddrinfo as a straight transform - so I can return new hostnames or ip addresses

e.g. in AWS I might want to transform an external hostname to an internal hostname using some regex - but I still want you to do the DNS resolution.

[technige]

That's an interesting idea. Maybe we should also fall back to getaddrinfo not just when no resolver is supplied but also if it returns nothing.

[eastlondoner]

What I'd like is something that works with

def my_resolver(socket_address):
         target_host, target_port = socket_address
         my_config = json.loads("/my-config-mount/latest.json")
         results = my_config[target_host]
         for x in results:
             yield x.host, x.port

given

### latest.json
{
  "dev": [
    {
      "host": "127.0.0.1"
      "port": 7687
    }
  ],
  "production": [
    {
      "host": "andy.neo4j.io"
      "port": 7000
    }
  ]
}

usage example
neo4j.driver(f"bolt+routing://dev")