Allow specifying port number of Google Cloud SQL Proxy client

[githubwua]

cloudsql-postgresql-plugin has a limitation that it only connects to private Cloud SQL Proxy on default port 5432. It is impossible to specify a different port number for the connection. As a result, this forces users to set up a separate VM instance running on port 5432 for every Postgres SQL instance.

If we have an option to specify a port number, we can connect to one single VM instance and have it run multiple copies of Google Cloud SQL Proxy client on different port numbers (e.g. 5432, 5434, 5436). This not only reduces running cost, but also management overhead. Please consider adding support for connecting to non-default port number. This would involve updating the following configuration field:

Connection Name: <proxy-ip>

to something like:

Connection Name: <proxy-ip>[:<port-number>]