Using ENV variables in Codegen

[kerimhudson]

The intention behind this PR is to try and introduce the ability for the codegen to utilise environment variables rather than a databaseUrl string value to make it easier to commit to repositories and setup different environments where necessary.

This PR is still a WIP as there's some things that likely need to be considered about how these environment variables will work in various situations (such as within a Next.js project where they should be prefixed with NEXT_PUBLIC).

Closes #1440

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Latest commit: 6adce23

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