complete support for `custom_serializable`

Currently, we allow users to customize the deserialization, see
https://github.com/simdjson/simdjson/blob/master/doc/basics.md#2-use-tag_invoke-for-custom-types-c20

But we are missing support for the equivalent with serialization.

```cpp
struct serialize_tag;
```

with a corresponding concept...

```cpp
 custom_serializable<T, value_type>
```

