Implemented phase 2 for the POC prototype in the Website service.

Services/Website/Sources/Library/Internal/Extensions/HTTPFields+Constants.swift

@@ -0,0 +1,8 @@
+import HTTPTypes
+
+extension HTTPFields {
+    /// The headers of a page rendered for its viewer: kept out of shared caches, and revalidated before reuse.
+    static let perViewer: Self = [
+        .cacheControl: "private, no-cache"
+    ]
+}