[plugins] Added support for being able to specify PGPORT to postgres plugin

[mohsenari]

Summary

Addresses #2773

How was it tested?

• prerequisite: add alias to compiled and built devbox binary (dx="path_to_devbox/dist/devbox")
• in 2 different directories named pg_test1 and pg_test2 run:
• • dx init
• • dx add postgresql
• • dx run -- initdb
• in pg_test2/devbox.json add "env": { "PGPORT": "5433" }
• • in both directories run dx services up
• • confirm both process composes run postgress on ports 5432 and 5433 without error

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[Lagoja]

Looks straightforward, g2g once the tests pass