feat: expose postgres with extension

[justinrubek]

This uses services-flake to run postgres locally, but also exposes the postgresql flake package when can be used to run postgres. To run it locally, use nix run .#services. This command will expect environment variables set: PRJ_DATA_HOME and PG1_PORT. For convenience, direnv will look in the .direnv/env directory for files that define environment variables (the file name is the variable name and file contents are its value).

This postgres has been verified to work with the extension:

➜ psql -h 127.0.0.1 -d array-example -p 5435
psql (15.6)
Type "help" for help.

array-example=# CREATE EXTENSION arrays;
WARNING:  creating sample data in the table 'vectors.data'.
CREATE EXTENSION
array-example=#