proxy: subzero integration in auth-broker (embedded data-api) (#12474)

## Problem
We want to have the data-api served by the proxy directly instead of
relying on a 3rd party to run a deployment for each project/endpoint.

## Summary of changes
With the changes below, the proxy (auth-broker) becomes also a
"rest-broker", that can be thought of as a "Multi-tenant" data-api which
provides an automated REST api for all the databases in the region.

The core of the implementation (that leverages the subzero library) is
in proxy/src/serverless/rest.rs and this is the only place that has "new
logic".

---------

Co-authored-by: Ruslan Talpa <ruslan.talpa@databricks.com>
Co-authored-by: Alexander Bayandin <alexander@neon.tech>
Co-authored-by: Conrad Ludgate <conrad@neon.tech>

proxy/src/cache/timed_lru.rs

@@ -204,6 +204,11 @@ impl<K: Hash + Eq + Clone, V: Clone> TimedLru<K, V> {
         self.insert_raw_ttl(key, value, ttl, false);
     }
 
+    #[cfg(feature = "rest_broker")]
+    pub(crate) fn insert(&self, key: K, value: V) {
+        self.insert_raw_ttl(key, value, self.ttl, self.update_ttl_on_retrieval);
+    }
+
     pub(crate) fn insert_unit(&self, key: K, value: V) -> (Option<V>, Cached<&Self, ()>) {
         let (_, old) = self.insert_raw(key.clone(), value);
 
@@ -214,6 +219,29 @@ impl<K: Hash + Eq + Clone, V: Clone> TimedLru<K, V> {
 
         (old, cached)
     }
+
+    #[cfg(feature = "rest_broker")]
+    pub(crate) fn flush(&self) {
+        let now = Instant::now();
+        let mut cache = self.cache.lock();
+
+        // Collect keys of expired entries first
+        let expired_keys: Vec<_> = cache
+            .iter()
+            .filter_map(|(key, entry)| {
+                if entry.expires_at <= now {
+                    Some(key.clone())
+                } else {
+                    None
+                }
+            })
+            .collect();
+
+        // Remove expired entries
+        for key in expired_keys {
+            cache.remove(&key);
+        }
+    }
 }
 
 impl<K: Hash + Eq, V: Clone> TimedLru<K, V> {