implemented prepared statement for postgres and sqlite

backends/postgres/src/postgres_dialect.cpp

@@ -1,7 +1,15 @@
 #include "postgres_dialect.hpp"
 
+#include "matador/sql/dialect_builder.hpp"
+
 [[maybe_unused]] const matador::sql::dialect* get_dialect() {
   using namespace matador::sql;
-  const static dialect d{};
+  const static dialect d = dialect_builder::builder()
+    .create()
+    .with_placeholder_func([](size_t index) {
+      return "$" + std::to_string(index);
+    })
+    .with_default_schema_name("public")
+    .build();
   return &d;
 }