feat: mandatory environment variables

[davelsio]

Summary

This PR makes some environment variables mandatory to start the server, removing defaults that could potentially be dangerous or lead hard to debug behavior.

Changes

• Treat ALLOW_ORIGIN and JWT_SECRET variables as mandatory and remove their defaults.
• Treat MAIL_* related variables as optional, but include no defaults.
• If MAIL_* variables are not set, the sendEmail service will throw the appropriate error.

Notes

• PR graphql-server-model-codegen#183 should be merged simultaneously.