feat: implement auth for mentor platform 🔐

[tomas-salgado]

Description ✏️

Implements authentication for the mentor platform, utilizing the same auth schema used for member-platform and admin-dashboard, except now pulling from a separate database table.

Type of Change 🐞

• Feature - A non-breaking change which adds functionality.
• Fix - A non-breaking change which fixes an issue.
• Refactor - A change that neither fixes a bug nor adds a feature.
• Documentation - A change only to in-code or markdown documentation.
• Tests - A change that adds missing unit/integration tests.
• Chore - A change that is likely none of the above.

Checklist ✅

• I have done a self-review of my code.
• I have manually tested my code (if applicable).
• I have added/updated any relevant documentation (if applicable).

[tomas-salgado]

Have only tested in local dev, which does not allow me to test the Google Auth and Slack Auth options.