Export contact tags and stages in CSV exports

This proposal has been discussed extensively on the Mautic forums at https://forum.mautic.org/t/include-contact-stage-and-tags-in-imports-and-exports/10262/20

The discussion includes 20 replies with community members confirming the need for this feature and discussing workarounds. Related discussions also exist at https://forum.mautic.org/t/export-contacts-with-tag-field/8582.

Currently, Mautic allows importing contacts with tags and stages, but does not provide the ability to export this critical information when exporting contacts to CSV format.

This project will implement comprehensive tag and stage export functionality in Mautic's contact export system with the following features:

1. Tags column in CSV exports: Add a "Tags" column to the contact export CSV that contains all tags assigned to each contact. Tags should be formatted in a way that supports re-importing (for example, comma-separated or pipe-separated values within the field).

2. Stage column in CSV exports: Add a "Stage" column to the contact export CSV that displays the current stage assigned to each contact, making lifecycle position visible in exported data.

3. Report builder integration: Ensure that tags and stages are available as selectable columns when building custom contact reports, allowing users to include or exclude these fields based on their specific reporting needs. As of Mautic 5.2, filtering by tags was added to reports, but the tags themselves still don't appear in the exported Excel file. 

4. Consistent field naming: Use field names and formatting that match the import process, enabling seamless export-modify-import workflows without requiring data transformation.

5. Segment export support: Ensure tags and stages are included when exporting contacts from specific segments, not just full database exports.

6. API endpoint enhancement: ensure consistency with tags and stages in the contact export API endpoints, ensuring consistent programmatic access to complete contact data.