loopdb: add postgres store

This commit adds a postgres store to the loopdb package.
Ths postgres migrator uses a replacer filesystem to replace the
sqlite types to postgres types in the migration.

loopdb/test_sqlite.go

@@ -0,0 +1,13 @@
+//go:build !test_db_postgres
+// +build !test_db_postgres
+
+package loopdb
+
+import (
+	"testing"
+)
+
+// NewTestDB is a helper function that creates an SQLite database for testing.
+func NewTestDB(t *testing.T) *SqliteSwapStore {
+	return NewTestSqliteDB(t)
+}