lnd+paymentsdb: introduce harness for the payment sql backend

We prepare the code for the sql payment backend. However no
payment db interface method for the sql backend is implemented
yet. This will be done in the following commits. They currently
use the embedded KVStore to satify the build environment.

payments/db/sql_store.go

@@ -0,0 +1,67 @@
+package paymentsdb
+
+import (
+	"fmt"
+
+	"github.com/lightningnetwork/lnd/sqldb"
+)
+
+// SQLQueries is a subset of the sqlc.Querier interface that can be used to
+// execute queries against the SQL payments tables.
+type SQLQueries interface {
+}
+
+// BatchedSQLQueries is a version of the SQLQueries that's capable
+// of batched database operations.
+type BatchedSQLQueries interface {
+	SQLQueries
+	sqldb.BatchedTx[SQLQueries]
+}
+
+// SQLStore represents a storage backend.
+type SQLStore struct {
+	// TODO(ziggie): Remove the KVStore once all the interface functions are
+	// implemented.
+	KVStore
+
+	cfg *SQLStoreConfig
+	db  BatchedSQLQueries
+
+	// keepFailedPaymentAttempts is a flag that indicates whether we should
+	// keep failed payment attempts in the database.
+	keepFailedPaymentAttempts bool
+}
+
+// A compile-time constraint to ensure SQLStore implements DB.
+var _ DB = (*SQLStore)(nil)
+
+// SQLStoreConfig holds the configuration for the SQLStore.
+type SQLStoreConfig struct {
+	// QueryConfig holds configuration values for SQL queries.
+	QueryCfg *sqldb.QueryConfig
+}
+
+// NewSQLStore creates a new SQLStore instance given an open
+// BatchedSQLPaymentsQueries storage backend.
+func NewSQLStore(cfg *SQLStoreConfig, db BatchedSQLQueries,
+	options ...OptionModifier) (*SQLStore, error) {
+
+	opts := DefaultOptions()
+	for _, applyOption := range options {
+		applyOption(opts)
+	}
+
+	if opts.NoMigration {
+		return nil, fmt.Errorf("the NoMigration option is not yet " +
+			"supported for SQL stores")
+	}
+
+	return &SQLStore{
+		cfg:                       cfg,
+		db:                        db,
+		keepFailedPaymentAttempts: opts.KeepFailedPaymentAttempts,
+	}, nil
+}
+
+// A compile-time constraint to ensure SQLStore implements DB.
+var _ DB = (*SQLStore)(nil)