16971 work package forms endpoint

[``* `#16971` Work package forms endpoint for API V3``](https://community.openproject.org/work_packages/16971)
Not touched by this PR but it would be good if it where changed to:
<pre>
case context[:ns]
when 'work_packages'
  ...
else
  fail API::Errors::InvalidRenderContext.new('Context does not exist!')
end
</pre>

You wouldn't even need the context_object variable then.