Add support for --failover-period flag #2386

We need to add a flag for configuring the underlying failover period for
disaster recovery when Proxy is configured with a DNS domain name.

The flag will just set the WithFailoverPeriod() Option for the Cloud SQL Go Connector under the hood.

The connector will poll for changes to the DNS name every 30 seconds by default.
You may configure the frequency of the connections using the option
WithFailoverPeriod(d time.Duration). When this is set to 0, the connector will
disable polling and only check if the DNS record changed when it is
creating a new connection.