Fix #1271 Add slack_file object to image block / block element classes

[seratch]

This pull request resolves #1271

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)
• [x] *slack-api--kotlin-extension** (Kotlin Extensions for Slack API Clients)
• [ ] slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (fdab0f2) 74.36% compared to head (245f07d) 74.30%.

Files	Patch %	Lines
.../model/kotlin_extension/block/ImageBlockBuilder.kt	50.00%	4 Missing :warning:
...ension/block/composition/SlackFileObjectBuilder.kt	80.00%	2 Missing :warning:
...api/model/block/composition/BlockCompositions.java	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1272 +/- ## ============================================ - Coverage 74.36% 74.30% -0.06% - Complexity 4125 4128 +3 ============================================ Files 443 444 +1 Lines 13080 13102 +22 Branches 1323 1323 ============================================ + Hits 9727 9736 +9 - Misses 2580 2594 +14 + Partials 773 772 -1 ```

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[seratch]

Thanks for the quick review!