Add domain name validation to Go Connector

[jackwotherspoon]

Add regex to validate domain name used is in proper format.

Similar to how we validate an instance connection name:

cloud-sql-go-connector/instance/conn_name.go

Lines 26 to 29 in 5ae88ea

	// Instance connection name is the format <PROJECT>:<REGION>:<INSTANCE>
	// Additionally, we have to support legacy "domain-scoped" projects
	// (e.g. "google.com:PROJECT")
	connNameRegex = regexp.MustCompile("([^:]+(:[^:]+)?):([^:]+):([^:]+)")

This will allow better error handling as there will be a different error for not being able
to resolve the DNS vs domain name being improperly formatted etc.

[jackwotherspoon]

Java Connector does this already:

https://github.com/GoogleCloudPlatform/cloud-sql-jdbc-socket-factory/blob/5e4cbb107a5ec9cf05908acd2b15093f296a02ea/core/src/main/java/com/google/cloud/sql/core/CloudSqlInstanceName.java#L53