Add user_list to feature_flag api

What does this MR do and why?

related issue: #417984 (closed)

Add strategy user_list to feature flags api

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Before	After

How to set up and validate locally

1. Create a project and remember the project ID: {{PROJECT_ID}}
2. Create a personal access token: {{ACCESS_TOKEN}}
3. Create a user list (replace {{PROJECT_ID}} and {{ACCESS_TOKEN}}):

   curl --location 'http://localhost:3000/api/v4/projects/{{PROJECT_ID}}/feature_flags_user_lists' \
   --header 'Content-Type: application/json' \
   --header 'Authorization: Bearer {{ACCESS_TOKEN}}' \
   --data '{
       "name": "my_user_list",
       "user_xids": "user1,user2,user3"
   }'

4. Remember the user list ID: {{USER_LIST_ID}}
5. Create a feature flag with the user list as strategy (replace {{PROJECT_ID}}, {{ACCESS_TOKEN}} and {{USER_LIST_ID}}):

   curl --location 'http://localhost:3000/api/v4/projects/{{PROJECT_ID}}/feature_flags' \
   --header 'Content-Type: application/json' \
   --header 'Authorization: Bearer {{ACCESS_TOKEN}}' \
   --data '{
       "name": "test",
       "version": "new_version_flag",
       "strategies": [
           {
               "name": "gitlabUserList",
               "user_list_id": {{USER_LIST_ID}}
           }
       ]
   }'

6. The feature flag should be successfully created

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