feat: Configure connections using DNS domain names

[hessjcg]

The dialer may be configured to use a DNS name to look up the instance
name instead of configuring the connector with the instance name directly.

Add a DNS TXT record for the Cloud SQL instance to a private DNS server
or a private Google Cloud DNS Zone used by your application. For example:

• Record type: TXT
• Name: prod-db.mycompany.example.com – This is the domain name used by the application
• Value: my-project:region:my-instance – This is the instance connection name
• Configure the dialer with the cloudsqlconn.WithDNSResolver() option.

Open a database connection using the DNS name:

//TODO

See also: the Cloud SQL Go Connector implementation

Part of #2043