Add "slack_file" properties to "image" blocks/elements under slack_sdk.models

[seratch]

The "image" blocks and block elements now can have "slack_file", which refers to an uploaded image file within Slack instead of "image_url", which must be a publicly hosted one. The Block Kit class representation in this SDK should add supports for these new options.

References:

• https://api.slack.com/reference/block-kit/blocks#image_fields
• https://api.slack.com/reference/block-kit/block-elements#image
• https://api.slack.com/reference/block-kit/composition-objects#slack_file

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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