Add documentdb postgres extension

[0dragosh]

Overview

I would like to have documentdb as a pre-installed extension in the official crunchy postgres image.

Use Case

I want to use FerretDB as a MongoDB interface, which requires documentdb to be installed and created in the postgres instance that’s backing it.

Desired Behavior

The extension binaries and libraries would come pre-installed in the crunchy postgres image, and I as a user would only have to run CREATE EXTENSION documentdb;