Use configured DNS name to lookup instance IP address

[kgala2]

When a custom DNS name is used to connect to a Cloud SQL instance, the dialer should first attempt to resolve the custom DNS name to an IP address and use that for the connection. If the lookup fails, the dialer should fall back to using the IP address from the instance metadata.

This change modifies the dialer to:

Use the configured resolver to look up the host's IP address.
Use the IP address from the A record of the custom DNS name if available.
Fall back to the IP address from the instance metadata if the A record is not available.
See also GoogleCloudPlatform/cloud-sql-go-connector#1053