feat: store migration setup (#794)

* feat: store migration setup

* comment

* changelog

x/feemarket/keeper/migrations.go

@@ -0,0 +1,13 @@
+package keeper
+
+// Migrator is a struct for handling in-place store migrations.
+type Migrator struct {
+	keeper Keeper
+}
+
+// NewMigrator returns a new Migrator.
+func NewMigrator(keeper Keeper) Migrator {
+	return Migrator{
+		keeper: keeper,
+	}
+}