Implement Migration Up/Down Logic for Validator DB (#8271)

* rollback logic

* implement up down logic

* begin down migration tests

* rollback works

* unset test

* remove iface

* gaz

* add comment

* fix ineff assign

* preston comment

* add progress

validator/db/BUILD.bazel

@@ -7,6 +7,7 @@ go_library(
         "alias.go",
         "cmd.go",
         "log.go",
+        "migrate.go",
         "restore.go",
     ],
     importpath = "github.com/prysmaticlabs/prysm/validator/db",