feat(auth): add NextAuth configuration and update environment templates

[maany]

This pull request introduces support for NextAuth authentication in the Rucio WebUI environment configuration. It adds new environment variables, updates the template and documentation to reflect these changes, and ensures that required authentication secrets are generated and validated automatically.

The most important changes include:

NextAuth Integration:

• Added support for NextAuth by introducing new environment variables (AUTH_SECRET, NEXTAUTH_SECRET, AUTH_TRUST_HOST, and NEXTAUTH_URL) to the environment template, with AUTH_SECRET and NEXTAUTH_SECRET being auto-generated if not provided. (tools/env-generator/src/api/base.ts [1] [2]; tools/env-generator/.env.template [3]
• Updated the .env template to include a new [auth] section with the relevant NextAuth variables, replacing the previous [session] section. (tools/env-generator/src/templates/.env.liquid tools/env-generator/src/templates/.env.liquidL16-R20)

Documentation Updates:

• Extended the README.md to document the new NextAuth-related environment variables, their defaults, and usage notes, including the requirement for NEXTAUTH_URL. (tools/env-generator/README.md tools/env-generator/README.mdR38-R50)

Validation Enhancements:

• Updated environment validation logic to require NEXTAUTH_URL as a mandatory variable. (tools/env-generator/src/api/base.ts tools/env-generator/src/api/base.tsL154-R156)