Add Duo Code Review mode resolver

What does this MR do and why?

Extracted from Refactor Duo Code Review mode resolution (!216690)

This MR adds a utility class to find in which mode Duo Code Review should run for a (user, container) pair, where the container can be either a group or a project.

This will help refactor the mode resolution later since we have to keep Classic Duo Code Review alive for the medium/long term.

How to set up and validate locally

Find a user and project that Duo Code Review should run in DAP mode and check:

user = User.find(...)
project = Project.find(...)
Ai::DuoCodeReview.enabled?(user:, container:) # => true
Ai::DuoCodeReview.mode(user:, container:)     # => :dap

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.