Adds backend support for adding custom attributes that propagate in a JWT

• Proto definitions and endpoints
• Database schema and migrations
• Template function as_sql_list
• JWT token integration

# Only allow access when restaurant_id user attribute is defined 
security: 
  access: {{ .user.restaurant_id }} IS NOT NULL
  row_filter: " restaurant_id = {{ .user.restaurant_id }}"

Example Policy ^

In another PR I'd like to add CLI commands for attribute management and bulk import / export

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch (I will once the rest of the requests are done)
• I'm proud of this work!