16972 add assignee responsible to wp form

[``* `#16972` Support assignee/responsible property on work package forms and PATCH endpoint``](https://community.openproject.org/work_packages/16972)

Includes https://community.openproject.org/work_packages/17426

- [x] Path service
The method call should be made in the endpoint/representer using this. But the structure in here could change to:
<pre>

module API
  module V3
    module Utilities
      module PathHelper
        include API::Utilities::UrlHelper
          class ApiV3Path

            ....

            def self.work_package_relations(id)
            ....
            end
  
            def self.work_package_watchers(id)
            ...
            end


            ....

          end
       def api_v3_paths
         ApiV3Path
       end
    end
end 
end

sorry about the indentation.