feat: Associate connected workspace agents with replicas (#4914)

This will enable displaying a graph that associates agents
to running replicas.

coderd/database/migrations/000070_deployment_graph.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE workspace_agents
+    DROP COLUMN last_connected_replica_id;

coderd/database/migrations/000070_deployment_graph.up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE workspace_agents
+    ADD COLUMN last_connected_replica_id uuid;