[FEAT] Add string similarity functions to PostGres backend

[vfrank66]

Is your proposal related to a problem?

I would like to use the postgres backend implementation which the codebase shows is not fully implemented due to a lack of similarity functions in postgres. I would like to add these similarity functions to enable full use of postgres backend.

Describe the solution you'd like

I would like to use the extension pg_similarity which includes all the similarity functions or I would like to inline create only the needed postgres similarity functions.

Describe alternatives you've considered

N/A

Additional context