Add shared_runners_minutes_limit to groups and users API

What does this MR do?

Add shared_runners_minutes_limit to groups and users API

Are there points in the code the reviewer needs to double check?

• I am not satisfied with the use of @declared_params, but I don't want to list everything either.
• There's a few stuffs are EE-only. In order to make it conflict less, we might need to make some updates to CE first.
• accepts_nested_attributes_for :namespace needs the id to update the relation, rather than creating it. This is annoying. Not sure if we should also move this to CE, considering that we might want to add some settings for the namespace in CE, too.
• Do we really want to expose the namespace? Or just make it one of the field for user?

Why was this MR needed?

From #1909 (closed)

We need the ability to change this value through the API, so we can pilot this from customers.gitlab.com, and enable the right limits after a customer purchases or changes a plan.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests
  • Added for this feature/bug

What are the relevant issue numbers?

#1909 (closed)

lib/api/entities.rb

@@ -38,6 +38,14 @@ module API
       expose :can_create_project?, as: :can_create_project
       expose :two_factor_enabled?, as: :two_factor_enabled
       expose :external
+
+      # EE-only
+      expose :namespace, using: 'API::Entities::UserNamespace'
+    end
+
+    # EE-only
+    class UserNamespace < Grape::Entity
+      expose :shared_runners_minutes_limit
     end
     class UserWithPrivateDetails < UserPublic
@@ -181,6 +189,9 @@ module API
     class GroupDetail < Group
       expose :projects, using: Entities::Project
       expose :shared_projects, using: Entities::Project
+
+      # EE-only
+      expose :shared_runners_minutes_limit
     end
     class RepoCommit < Grape::Entity