Add multimodal support for Linear

[codegen-sh]

This PR adds multimodal support for Linear, allowing the codegen bot to handle file attachments from Linear issues.

Changes:

1. Enhanced LinearEvent model to include attachments

2. Added new models for file attachments and upload responses

3. Added methods to the LinearClient class:

   • get_issue_attachments: Fetches attachments for an issue
   • download_attachment: Downloads a file attachment
   • request_upload_url: Requests a pre-signed URL for file upload
   • upload_file: Uploads a file to Linear

4. Updated the Linear event handler class:

   • Added a client property to access the Linear API
   • Enhanced event handling to automatically fetch attachments for issues
   • Added methods to download and upload files

These changes enable the codegen bot to:

• Automatically fetch file attachments when handling Linear events
• Download attachments to local storage for processing
• Upload files to Linear when needed

This implementation follows a similar pattern to the Slack multimodal support, but is adapted for Linear's specific API requirements.

[tawsifkamal]

call this raw_content