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

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

从 slack_sdk.models 开始，重点关注图像块构建器以及其 image_url 值的表示方式。使用私下上传的 Slack 文件验证其行为，并在该块能够引用该文件、而无需将其公开或手动提取公共 URL 时，视为工作完成。