storcon: implement safekeeper_migrate handler (#11849)

This PR implements a safekeeper migration algorithm from RFC-035


https://github.com/neondatabase/neon/blob/main/docs/rfcs/035-safekeeper-dynamic-membership-change.md#change-algorithm

- Closes: https://github.com/neondatabase/neon/issues/11823

It is not production-ready yet, but I think it's good enough to commit
and start testing.

There are some known issues which will be addressed in later PRs:
- https://github.com/neondatabase/neon/issues/12186
- https://github.com/neondatabase/neon/issues/12187
- https://github.com/neondatabase/neon/issues/12188
- https://github.com/neondatabase/neon/issues/12189
- https://github.com/neondatabase/neon/issues/12190
- https://github.com/neondatabase/neon/issues/12191
- https://github.com/neondatabase/neon/issues/12192

## Summary of changes
- Implement `tenant_timeline_safekeeper_migrate` handler to drive the
migration
- Add possibility to specify number of safekeepers per timeline in tests
(`timeline_safekeeper_count`)
- Add `term` and `flush_lsn` to `TimelineMembershipSwitchResponse`
- Implement compare-and-swap (CAS) operation over timeline in DB for
updating membership configuration safely.
- Write simple test to verify that migration code works

libs/pageserver_api/src/controller_api.rs

@@ -546,6 +546,11 @@ pub struct TimelineImportRequest {
     pub sk_set: Vec<NodeId>,
 }
 
+#[derive(serde::Serialize, serde::Deserialize, Clone)]
+pub struct TimelineSafekeeperMigrateRequest {
+    pub new_sk_set: Vec<NodeId>,
+}
+
 #[cfg(test)]
 mod test {
     use serde_json;