block/type/image should allow image_url to reference an uploaded file to Slack without having to make it public

(Describe your issue and goal here)

This should work:

1. upload an image with client.files_upload_v2(), do not want it to be visible in a channel
2. chat_postMessage(blocks=type: image, image_url: file.private_url ...)

This currently doesn't work unless the file/image is made public via client.files_sharedPublicURL(), but enabling public files is not an option in many organisations. Additionally one shouldn't have to construct urls by extracting pub_secret from file/permalink_public.

Given that slack client can show uploaded images (even when they are not public), it should also work with blocks/type/image.

Example discussion/solution (with making the file public) - https://stackoverflow.com/questions/58186399/how-to-create-a-slack-message-containing-an-uploaded-image

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

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Bắt đầu tại slack_sdk.models, tập trung vào image block builder và cách giá trị image_url của nó được biểu diễn. Xác minh hành vi bằng một tệp Slack được tải lên ở chế độ riêng tư và coi công việc là hoàn tất khi block có thể tham chiếu đến tệp đó mà không cần công khai tệp hoặc trích xuất thủ công một URL công khai.