Improve nuget/rubygems package registries (#34741)

1. Add some missing (optional) fields for nuget v2, and sort the fields
to make it easier to maintain
2. Add missing "platform" for rubygems: `VERSION-PLATFORM` and
`VERSION_PLATFORM`

Co-authored-by: Giteabot <teabot@gitea.io>

modules/packages/container/const.go

@@ -0,0 +1,11 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package container
+
+const (
+	ContentTypeDockerDistributionManifestV2 = "application/vnd.docker.distribution.manifest.v2+json"
+
+	ManifestFilename = "manifest.json"
+	UploadVersion    = "_upload"
+)