Add duo_evaluation_ready feature flag

What does this MR do and why?

This MR adds a feature flag for One click Duo Chat evaluation in MRs (#497305 - closed). This duo_evaluation_ready is a signal that the GDK instance is ready for evaluation. This is mainly used in https://gitlab.com/gitlab-org/modelops/ai-model-validation-and-research/ai-evaluation/evaluation-runner.

[4] pry(main)> Feature.enable(:duo_evaluation_ready)

shinya@shinya-XPS-15-9530:~/gdk/gitlab$ curl --header "PRIVATE-TOKEN: [REDACTED]" "http://gdk.test:3000/api/v4/features" | jq --arg name "duo_evaluation_ready" '.[] | select(.name == $name).state'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   433  100   433    0     0  14034      0 --:--:-- --:--:-- --:--:-- 14433
"on"

How this feature flag is used

When evaluation runner kicks off an evaluation pipeline which launches a remote GDK instance, it executes wait-for-instance job to make sure the instance is ready for evaluation, example. We're currently checking the HTTP status from GitLab-Rails url, however this technically returns 200 even before the post-deployment script is complete. We enable the duo_evaluation_ready feature flag at the end of the post-deployment script, and change the wait-for-instance job to request to http://gdk.test:3000/api/v4/features endpoint to get the state of the FF. So that the evaluation won't start until the GDK is ready.

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