Add authenticate v2 endpoint

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What does this MR do and why?

This MR adds the authenticate Conan V2 endpoint.

Changelog: added

References

• Implement Required Endpoints for Conan v2 Revis... (#519741 - closed)
• #333033 (comment 2060136937)

How to set up and validate locally

1. Switch the conan_package_revisions_support feature flag on.

   For GDK, open rails console (docs):

   gdk rails console

   Enable the feature flag:

   # Enable
   Feature.enable(:conan_package_revisions_support)
   
   # Verify
   Feature.enabled?(:conan_package_revisions_support)

2. Generate a base64-encoded Basic Auth token

   # Replace with your username and PAT
   echo -n "<username>:<personal_access_token>"|base64

3. Call the authenticate endpoint

   curl --request GET \
   --url 'http://localhost:3000/api/v4/projects/<project_id>/packages/conan/v2/users/authenticate' \
   --header 'Authorization: Basic <YOUR-BASE64-TOKEN>'

   that should return a JWT token like:

   eyJhbGciOiJIUzI1NiIiheR5cCI6IkpXVCJ9.eyJhY2Nlc3NfdG9rZW4iOjMyMTQyMzAsqaVzZXJfaWQiOjQwNTkyNTQsImp0aSI6IjdlNzBiZTNjLWFlNWQtNDEyOC1hMmIyLWZiOThhZWM0MWM2OSIsImlhd3r1MTYxNjYyMzQzNSwibmJmIjoxNjE2NjIzNDMwLCJleHAiOjE2MTY2MjcwMzV9.QF0Q3ZIB2GW5zNKyMSIe0HIFOITjEsZEioR-27Rtu7E

4. Validate the token

   Use the response from step 3 instead of <authenticate_token>

   curl --request GET \
        --header "Authorization: Bearer <authenticate_token>" \
        --url "http://localhost:3000/api/v4/projects/<project_id>/packages/conan/v2/users/check_credentials"

   that should return:

   ok

/cc @oceane_scania

Related to #519741 (closed)