sqldb: add the sqldb package

This commit provides the scaffolding for using the new sql stores.
The new interfaces, structs and methods are in sync with other projects
like Taproot Assets.

- Transactional Queries: the sqldb package defines the interfaces required
to execute transactional queries to our storage interface.

- Migration Files Embedded: the migration files are embedded into the binary.

- Database Migrations: I kept the use of 'golang-migrate' to ensure our
codebase remains in sync with the other projects, but can be changed.

- Build Flags for Conditional DB Target: flexibility to specify our database
target at compile-time based on the build flags in the same way we do
with our kv stores.

- Update modules: ran `go mod tidy`.

sqldb/postgres_test.go

@@ -0,0 +1,13 @@
+//go:build test_db_postgres
+// +build test_db_postgres
+
+package sqldb
+
+import (
+	"testing"
+)
+
+// NewTestDB is a helper function that creates a Postgres database for testing.
+func NewTestDB(t *testing.T) *PostgresStore {
+	return NewTestPostgresDB(t)
+}