feat: add upload and download support for work item attachments

[akhil-vamshi-konam]

Description

Adds helper methods for handling work item attachments end-to-end:

• upload_from_bytes() to upload attachment content directly from memory.
• get_download_url() to retrieve a presigned download URL for an attachment.
• Handles the complete attachment upload flow:
  1. Create attachment record
  2. Upload file to S3 using the presigned URL
  3. Mark attachment as uploaded

Also bumps SDK version from 0.2.15 to 0.2.16.

Type of Change

• Feature (non-breaking change which adds functionality)

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ec58a64e-95c8-4dc5-80e7-e481f91adb69

📥 Commits

Reviewing files that changed from the base of the PR and between 491a923 and f53aee5.

📒 Files selected for processing (2)

• plane/api/work_items/attachments.py
• pyproject.toml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat-attachments

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