Implement package metadata endpoint on the group level

...g/feature_flags/development/npm_group_level_endpoints.yml

doc/api/packages/npm.md

doc/user/packages/npm_registry/index.md

lib/api/api.rb

lib/api/helpers/packages/npm.rb

lib/api/npm_group_packages.rb

lib/api/npm_instance_packages.rb

lib/api/npm_project_packages.rb

spec/lib/api/helpers/packages/npm_spec.rb

spec/requests/api/npm_group_packages_spec.rb

...red_examples/requests/api/npm_packages_shared_examples.rb