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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Start at slack_sdk.models, focusing on the image block builder and how its image_url value is represented. Verify the behavior with a privately uploaded Slack file, and consider the work complete when the block can reference that file without making it public or manually extracting a public URL.