Implement offset support (#102)

As per discussions in [1].

[1] https://github.com/getml/sqlgen/issues/100

tests/sqlite/test_offset.cpp

@@ -0,0 +1,50 @@
+#include <gtest/gtest.h>
+
+#include <rfl.hpp>
+#include <rfl/json.hpp>
+#include <sqlgen.hpp>
+#include <sqlgen/sqlite.hpp>
+#include <vector>
+
+namespace test_offset {
+
+struct Person {
+  sqlgen::PrimaryKey<uint32_t> id;
+  std::string first_name;
+  std::string last_name;
+  int age;
+};
+
+TEST(sqlite, test_offset) {
+  const auto people1 = std::vector<Person>(
+      {Person{
+           .id = 0, .first_name = "Homer", .last_name = "Simpson", .age = 45},
+       Person{.id = 1, .first_name = "Bart", .last_name = "Simpson", .age = 10},
+       Person{.id = 2, .first_name = "Lisa", .last_name = "Simpson", .age = 8},
+       Person{
+           .id = 3, .first_name = "Maggie", .last_name = "Simpson", .age = 0},
+       Person{
+           .id = 4, .first_name = "Hugo", .last_name = "Simpson", .age = 10}});
+
+  const auto conn = sqlgen::sqlite::connect();
+
+  sqlgen::write(conn, people1);
+
+  using namespace sqlgen;
+  using namespace sqlgen::literals;
+
+  // Note: SQLite does not support offset without limit.
+  const auto query =
+          sqlgen::read<std::vector<Person>> | order_by("id"_c) | limit(5) | offset(3);
+
+  const auto sql = sqlite::to_sql(query);
+
+  const auto people2 = query(conn).value();
+
+  const std::string expected =
+      R"([{"id":3,"first_name":"Maggie","last_name":"Simpson","age":0},{"id":4,"first_name":"Hugo","last_name":"Simpson","age":10}])";
+
+  EXPECT_EQ(rfl::json::write(people2), expected);
+}
+
+}  // namespace test_offset