safekeeper: add term_bump endpoint.

When walproposer observes now higher term it restarts instead of
crashing whole compute with PANIC; this avoids compute crash after
term_bump call. After successfull election we're still checking
last_log_term of the highest given vote to ensure basebackup is good,
and PANIC otherwise.

It will be used for migration per
035-safekeeper-dynamic-membership-change.md
and
https://github.com/neondatabase/docs/pull/21

ref https://github.com/neondatabase/neon/issues/8700

libs/safekeeper_api/src/models.rs

@@ -60,3 +60,16 @@ pub struct TimelineCopyRequest {
     pub target_timeline_id: TimelineId,
     pub until_lsn: Lsn,
 }
+
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct TimelineTermBumpRequest {
+    /// bump to
+    pub term: Option<u64>,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct TimelineTermBumpResponse {
+    // before the request
+    pub previous_term: u64,
+    pub current_term: u64,
+}