Support custom extensions

Problem to solve

There are a few extensions that I use in production which I would like to use in a managed StackGres cluster environment such as ZomboDB. ZomboDB needs to be custom compiled by those that want to use it, thus it would not make sense to add it to your extensions list (though if you do want to find a way to add it as a supported extension, I would be very happy). There are also more obscure extensions i have used such as pgsql-postal. If there was a documented way to package extensions for StackGres to use, this would be incredibly useful.

Further details

In addition to my own specific needs, what if someone has written a custom postgres extension and would like to use it in a StackGres cluster? What if someone has modified an existing extension and wants to use it?

Proposal

What I propose it to address these issues by creating a documented way for users of StackGres to package custom extensions for use in StackGres clusters.

Testing

While the method of packaging extensions should be clear and concise, it should be made clear that extensions packaged by individuals for private or independent use would not be supported by the StackGres team. The only issues with custom extensions that should be addressed would be issues with the packaging and system itself, not the extensions.

Acceptance Criteria

This issue could be accepted if there is a documented way to utilize custom extensions in StackGres.

Links / references

current information on the utilization of existing extensions: https://stackgres.io/doc/latest/reference/crd/sgcluster/