[plugins] Added support for being able to specify PGPORT to postgres plugin (#2774)

## 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

## Community Contribution License

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[Community Contribution
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