Add dummy lsn lease http and page service APIs (#7815)

We want to introduce a concept of temporary and expiring LSN leases.
This adds both a http API as well as one for the page service to obtain
temporary LSN leases.

This adds a dummy implementation to unblock integration work of this
API. A functional implementation of the lease feature is deferred to a
later step.

Fixes #7808

Co-authored-by: Joonas Koivunen <joonas@neon.tech>

libs/pageserver_api/src/models.rs

@@ -162,6 +162,22 @@ impl std::fmt::Debug for TenantState {
     }
 }
 
+/// A temporary lease to a specific lsn inside a timeline.
+/// Access to the lsn is guaranteed by the pageserver until the expiration indicated by `valid_until`.
+#[serde_as]
+#[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
+pub struct LsnLease {
+    #[serde_as(as = "SystemTimeAsRfc3339Millis")]
+    pub valid_until: SystemTime,
+}
+
+serde_with::serde_conv!(
+    SystemTimeAsRfc3339Millis,
+    SystemTime,
+    |time: &SystemTime| humantime::format_rfc3339_millis(*time).to_string(),
+    |value: String| -> Result<_, humantime::TimestampError> { humantime::parse_rfc3339(&value) }
+);
+
 /// The only [`TenantState`] variants we could be `TenantState::Activating` from.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 pub enum ActivatingFrom {

libs/pageserver_api/src/shard.rs

@@ -559,6 +559,14 @@ impl ShardIdentity {
         }
     }
 
+    /// Obtains the shard number and count combined into a `ShardIndex`.
+    pub fn shard_index(&self) -> ShardIndex {
+        ShardIndex {
+            shard_count: self.count,
+            shard_number: self.number,
+        }
+    }
+
     pub fn shard_slug(&self) -> String {
         if self.count > ShardCount(0) {
             format!("-{:02x}{:02x}", self.number.0, self.count.0)