use subfolders for attachments

[sowjanyakch]

Resolve #6034

For NC34, we use subfolders for attachments per conversation.

1. We need to get the draft folder from the endpoint, predicted filenames in case of conflict from the endpoint ....../attachment/folder endpoint
2. Upload attachments to the draft folder using the predicted filenames.
3. Call ....../attachment endpoint to finalise the upload and to post a chat message.

🚧 TODO

• ...

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/24994930982/artifacts/6660687770
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[github-actions]

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[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/25391802249/artifacts/6813658464
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
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[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/25449003837/artifacts/6836474927
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[github-actions]

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[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/25450704773/artifacts/6837404172
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