Newsletter: include more info in export CSV

[simison]

Currently Export only includes emails, no other info.

Let's include more data for easier migration to alternative services, for subscriber profiles we currently have:

Include at least:

• Newsletter categories (if enabled)
• Paid/free subscriber (if enabled)
• Subscription date
• WP.com username
• Reader/email subscription

Optionally, we have:

• HTML/text email format
• Instant/daily/weekly schedule
• Click rate
• Open rate
• Emails sent
• Country

All these also let customers do their own analysis on their chosen software like Google Sheets.

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] A8C Marketing & Sales: The issue relates to improving the export functionality, which is critical for marketing and sales operations dealing with subscriber data.
• [Feature] Subscriptions: The request to include more subscriber information in the export is directly connected to subscription management features.
• [Feature] Stats Data: Including metrics such as click rate and open rate would contribute to analytics capabilities, aligning with the stats data feature.
• [Feature] Mailchimp: The suggestion relates to the migration to alternative services, such as Mailchimp, which involves exporting subscriber information.