Add experimental C++26 reflection ORM

[1ukidev]

This PR aims to implement a new way of using the ORM (orm::ReflectiveMapper) through the compile-time reflection introduced in C++26. I have been working on this feature for personal use for some time, and I thought it would be interesting to share it here. I am not sure how open the project is to adopting newer C++ standards.

Thanks to reflection annotations, it is currently possible to write code such as:

namespace refl = drogon::orm::reflection;

struct [[=refl::Table{"users"}]] User
{
    [[=refl::PrimaryKey{}, =refl::AutoGenerated{}]]
    std::optional<std::uint64_t> id;

    [[=refl::Column{"user_name"}]]
    std::string name;

    std::optional<std::string> email;
}

All metadata is generated statically at compile time, including the query strings used for retrieval, insertion, counting, updating, and deletion. This practically eliminates the need for utilities such as drogon_ctl.

The workflow with orm::ReflectiveMapper is as simple as:

const auto client = DbClient::newSqlite3Client("filename=:memory:", 1);

ReflectiveMapper<User> mapper{client};

User user;
user.name = "Ada";
user.email = "ada@example.test";

mapper.insert(user);

const auto nameColumn = mapper.columnName<^^User::name>();
auto loaded = mapper.findOne(
    Criteria{nameColumn, CompareOperator::EQ, std::string{"Ada"}});

A example is available in examples/orm_reflection/main.cc. For now, only synchronous and callback-based database communication is supported, but other approaches can be discussed in the future.

Currently, only GCC 16 provides official experimental support for reflection, which may require additional caution when using it in production. I'm marking this as a draft for now. Feel free to ask if you have any questions.