storcon: notify cplane on safekeeper membership change (#12390)

## Problem
We don't notify cplane about safekeeper membership change yet. Without
the notification the compute needs to know all the safekeepers on the
cluster to be able to speak to them. Change notifications will allow to
avoid it.

- Closes: https://github.com/neondatabase/neon/issues/12188

## Summary of changes
- Implement `notify_safekeepers` method in `ComputeHook`
- Notify cplane about safekeepers in `safekeeper_migrate` handler.
- Update the test to make sure notifications work.

## Out of scope
- There is `cplane_notified_generation` field in `timelines` table in
strocon's database. It's not needed now, so it's not updated in the PR.
Probably we can remove it.
- e2e tests to make sure it works with a production cplane

test_runner/regress/test_safekeeper_migration.py

@@ -32,10 +32,13 @@ def test_safekeeper_migration_simple(neon_env_builder: NeonEnvBuilder):
     )
 
     ep = env.endpoints.create("main", tenant_id=env.initial_tenant)
-    # We specify all safekeepers, so compute will connect to all of them.
-    # Only those from the current membership configuration will be used.
-    # TODO(diko): set only current safekeepers when cplane notify is implemented.
-    ep.start(safekeeper_generation=1, safekeepers=[1, 2, 3])
+
+    mconf = env.storage_controller.timeline_locate(env.initial_tenant, env.initial_timeline)
+    assert mconf["new_sk_set"] is None
+    assert len(mconf["sk_set"]) == 1
+    assert mconf["generation"] == 1
+
+    ep.start(safekeeper_generation=1, safekeepers=mconf["sk_set"])
     ep.safe_psql("CREATE EXTENSION neon_test_utils;")
     ep.safe_psql("CREATE TABLE t(a int)")
 
@@ -58,7 +61,16 @@ def test_safekeeper_migration_simple(neon_env_builder: NeonEnvBuilder):
 
     assert ep.safe_psql("SELECT * FROM t") == [(i,) for i in range(1, 4)]
 
+    # 1 initial generation + 2 migrations on each loop iteration.
+    expected_gen = 1 + 2 * 3
+
+    mconf = env.storage_controller.timeline_locate(env.initial_tenant, env.initial_timeline)
+    assert mconf["generation"] == expected_gen
+
+    assert ep.safe_psql("SHOW neon.safekeepers")[0][0].startswith(f"g#{expected_gen}:")
+
+    # Restart and check again to make sure data is persistent.
     ep.stop()
-    ep.start(safekeeper_generation=1, safekeepers=[1, 2, 3])
+    ep.start(safekeeper_generation=1, safekeepers=[3])
 
     assert ep.safe_psql("SELECT * FROM t") == [(i,) for i in range(1, 4)]