mmaprototype: move rebalanceStores to its own file

Epic: CRDB-55052

Author: tbg

pkg/kv/kvserver/allocator/mmaprototype/BUILD.bazel

@@ -6,6 +6,7 @@ go_library(
         "allocator.go",
         "allocator_state.go",
         "cluster_state.go",
+        "cluster_state_rebalance_stores.go",
         "constraint.go",
         "constraint_matcher.go",
         "doc.go",