Handle db extension plpgsql the same way same btree_gist is managed

We need the database extension btree_gist and ensure that it is installed, but for plpgsql this is not followed

btree_gist

• created in structure.sql
• documented as needed for external database and replication

there is no reference to plpgsql which is available by default in Omnibus embedded database but not guaranteed in external databases. (Azure)

If this extension is not available when the customer upgrades; database migration will fail with

PG::UndefinedObject: ERROR:  language "plpgsql" does not exist and HINT:  Use CREATE EXTENSION to load the language into the database.

Documentation update is via Doc update - Add extension plpgsql is required