Forward pageserver connection string from compute to safekeeper

This is needed for implementation of tenant rebalancing. With this
change safekeeper becomes aware of which pageserver is supposed to be
used for replication from this particular compute.

test_runner/batch_others/test_wal_acceptor.py

@@ -340,6 +340,7 @@ class ProposerPostgres:
                 "synchronous_standby_names = 'walproposer'\n",
                 f"zenith.zenith_timeline = '{self.timeline_id}'\n",
                 f"zenith.zenith_tenant = '{self.tenant_id}'\n",
+                f"zenith.page_server_connstring = ''\n",
                 f"wal_acceptors = '{wal_acceptors}'\n",
             ])