Provide a Cloud SQL extension

[loicmathieu]

Using Cloud SQL is not that simple as it needs additional libraries, configuration and native support.
All these are explained in the guide: https://quarkus.io/guides/deploying-to-google-cloud

But when things changed, a guide change will not make it change to everybody via a simple quarkus update.
See for ex: quarkusio/quarkus#43901 (comment)

Having an extension would makes things easier, this would of course needs one extension by supported databases.