Add Slack file upload APIs

[wbssbw]

Summary

Implements support for Slack's external file upload workflow, which allows modules to upload files to Slack channels using the files.getUploadURLExternal, file upload, and files.completeUploadExternal APIs. This provides a complete three-step file upload flow: obtaining an upload URL, uploading the file content, and finalizing the upload to share in channels.

Changes

• STL additions: Added get_file_upload_url, upload_file, and complete_file_upload functions in plaid-stl/src/slack/mod.rs
• Runtime implementation: Implemented get_file_upload_url, upload_file, and complete_file_upload methods in runtime/plaid/src/apis/slack/api.rs with proper error handling and HTTP client integration

[Copilot]

Pull request overview

This PR implements Slack's external file upload workflow, enabling modules to upload files to Slack channels through a three-step process: obtaining an upload URL, uploading file content, and completing the upload.

Key Changes:

• Added three new STL functions (get_file_upload_url, upload_file, complete_file_upload) to support the complete file upload flow
• Implemented runtime methods with proper HTTP request handling and error management for each upload step
• Registered new functions in the runtime's function registry with appropriate test mode restrictions

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
runtime/plaid/src/functions/api.rs	Registers the new Slack file upload functions in the runtime's function registry
runtime/plaid/src/apis/slack/api.rs	Implements the runtime methods for handling file upload requests with proper HTTP clients and error handling
runtime/plaid-stl/src/slack/mod.rs	Defines STL public API functions and data structures for the file upload workflow

Comments suppressed due to low confidence (1)

runtime/plaid/src/apis/slack/api.rs:1

• The documentation comment for length has inconsistent punctuation compared to other fields. The bot field comment doesn't end with a period, while length and filename do. Consider removing the period from length and filename comments for consistency, or add one to the bot comment.

use std::sync::Arc;

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[obelisk]

I don't love us passing information to the rule which we then need to re-verify. Should this just be abstracted behind a single API that uploads a file? What would be the use case for rules to do these individually?