Thanks for the response. There steps are you provided are for an installation where the Postgres database is running on the local machine, therefore the required jar file can be added locally. In my installation the database is managed via Google Cloud's CloudSQL function. Due to this I am not able to add the jar file directly to the database lib folder.
On further investigation I have found that the PL/Java extension is not available from Google but a request has been raised to have it added (https://issuetracker.google.com/issues/148964024). I believe you wont be able to run Pega 8.3 and above on CloudSQL until this is addressed.