Add optional query parameter to return all translated fields in resource APIs

[nrjadkry]

If a metadata field is configured to support translations, we want all translated fields to be optionally returned in the API response for any resource type, such as datasets/, documents/, or maps/.

A new optional query parameter, include_i18n, will be introduced. When set to true, the response should include all available translations for translatable fields, for example:

title: (translated title as we have now),
title_en: (engligh title),
title_ru: (russian),
(...)

This feature can be implemented via the MultiLangViewMixin which already handles localized field resolution

Note: Default behavior must remain unchanged for backward compatibility, and translations should only be included when the include_i18n parameter is explicitly requested.