Add slack_file object to image block / block element classes

[seratch]

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 [ ])

• [ ] bolt (Bolt for Java)
• [ ] bolt-{sub modules} (Bolt for Java - optional modules)
• [ ] slack-api-client (Slack API Clients)
• [x] slack-api-model (Slack API Data Models)
• [ ] *slack-api--kotlin-extension** (Kotlin Extensions for Slack API Clients)
• [ ] slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please make sure if this topic is specific to this SDK. For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. :bow:

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