feat: Support configure connections using DNS

[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:

db, err := sql.Open(
    "cloudsql-mysql",
    "myuser:mypass@cloudsql-mysql(prod-db.mycompany.example.com)/mydb",
)

Part of #842