16971 work package forms endpoint

[``* `#16971` Work package forms endpoint for API V3``](https://community.openproject.org/work_packages/16971)
Just a matter of taste, so feel free to ignore. I think it would be easier to read if it would be something like:

<pre>
context = ::API::V3::Utilities::ResourceLinkParser.parse(params[:context])

if context.nil?
 fail API::Errors::InvalidRenderContext.new('No context found.')
elsif !SUPPORTED_CONTEXT_NAMESPACES.include? context[:ns]
  fail API::Errors::InvalidRenderContext.new('Unsupported context found.')
else
  context
end