Adds enable_member_management setting to api response

What does this MR do and why?

Adds enable_member_management setting to api response

EE: true Changelog: added

ref: #470551 (closed)

MR acceptance checklist

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Screenshots or screen recordings

enable_member_promotion_management-api

How to set up and validate locally

1. On master, using your access-token (as an administrator) (document)

   curl --header "PRIVATE-TOKEN: glpat-somerandomvalue" "http://localhost:3000/api/v4/application/settings" | grep enable_member_promotion_management

   should return empty

2. Attempt to update the value should return error

   curl --request PUT --header "PRIVATE-TOKEN: glpat-somerandomvalue" "http://localhost:3000/api/v4/application/settings?enable_member_promotion_management=true"
   
   {"error":"abuse_notification_email... are missing, at least one parameter must be provided"}

3. On this branch,

   curl --header "PRIVATE-TOKEN: glpat-somerandomvalue" "http://localhost:3000/api/v4/application/settings" | grep enable_member_promotion_management

   should return a value.

4. You should also be able to update the value using curl

   curl --request PUT --header "PRIVATE-TOKEN: glpat-somerandomvalue" "http://localhost:3000/api/v4/application/settings?enable_member_promotion_management=true"

   should update and return the modified application setting.