Add support for GCP CloudSQL Native Connection

[eugenepaniot]

This PR addes support for native connection to Google CloudSQL Mysql Instances (without any extra auth proxy). See https://github.com/petoju/terraform-provider-mysql/issues/40

Usage example:

provider "mysql" {
  endpoint = "cloudsql://my-project:us-central1:my-mysql-instance"
  username = "root"
  password = "password"
}

my-project:us-central1:my-mysql-instance is sql_database_instance.connection_name

[petoju]

Thanks!