codify "platform" into resolution logic, server-fn/fullstack overhaul (#4596)

overhaul dioxus fullstack using a new implementation of server_fn

This PR is a complete rewrite of dioxus fullstack designed to improve compatibility with axum and to fix a number of compilation errors based on feature flag unification plumbing through macros.

packages/fullstack-server/src/isrg/freshness.rs

@@ -0,0 +1,82 @@
+use std::time::Duration;
+
+use chrono::{DateTime, Utc};
+
+/// Information about the freshness of a rendered response
+#[derive(Debug, Clone, Copy)]
+pub struct RenderFreshness {
+    /// The age of the rendered response
+    age: u64,
+    /// The maximum age of the rendered response
+    max_age: Option<u64>,
+    /// The time the response was rendered
+    timestamp: DateTime<Utc>,
+}
+
+impl RenderFreshness {
+    /// Create new freshness information
+    pub(crate) fn new(age: u64, max_age: u64, timestamp: DateTime<Utc>) -> Self {
+        Self {
+            age,
+            max_age: Some(max_age),
+            timestamp,
+        }
+    }
+
+    /// Create new freshness information with only the age
+    pub(crate) fn new_age(age: u64, timestamp: DateTime<Utc>) -> Self {
+        Self {
+            age,
+            max_age: None,
+            timestamp,
+        }
+    }
+
+    /// Create new freshness information from a timestamp
+    pub(crate) fn created_at(timestamp: DateTime<Utc>, max_age: Option<Duration>) -> Self {
+        Self {
+            age: timestamp
+                .signed_duration_since(Utc::now())
+                .num_seconds()
+                .unsigned_abs(),
+            max_age: max_age.map(|d| d.as_secs()),
+            timestamp,
+        }
+    }
+
+    /// Create new freshness information at the current time
+    pub fn now(max_age: Option<Duration>) -> Self {
+        Self {
+            age: 0,
+            max_age: max_age.map(|d| d.as_secs()),
+            timestamp: Utc::now(),
+        }
+    }
+
+    /// Get the age of the rendered response in seconds
+    pub fn age(&self) -> u64 {
+        self.age
+    }
+
+    /// Get the maximum age of the rendered response in seconds
+    pub fn max_age(&self) -> Option<u64> {
+        self.max_age
+    }
+
+    /// Get the time the response was rendered
+    pub fn timestamp(&self) -> DateTime<Utc> {
+        self.timestamp
+    }
+
+    /// Write the freshness to the response headers.
+    pub fn write(&self, headers: &mut http::HeaderMap<http::HeaderValue>) {
+        let age = self.age();
+        headers.insert(http::header::AGE, age.into());
+        if let Some(max_age) = self.max_age() {
+            headers.insert(
+                http::header::CACHE_CONTROL,
+                http::HeaderValue::from_str(&format!("max-age={}", max_age)).unwrap(),
+            );
+        }
+    }
+}