Validate env vars with Zod

[infomiho]

This PR will enable us to have strict env vars validation with Zod for Wasp's internal env vars. Also, it will enable users to define their own env var validation. Wasp will use the Zod schemas to output errors for missing env vars.

Left to do

• [x] After the strict null checks is merged, make sure that the types are correct #2360
• [x] Make sure all env vars in our code are used through the env object and not directly
• [ ] Experiment with enabling users to add their own env vars validation https://github.com/wasp-lang/wasp/pull/2363
• [ ] Update docs
• [x] Fix keycloak.env.KEYCLOAK_REALM_URL leftover

Closes #1067