feat(block-kit): add data table block example

[zimeg]

Summary

Adds a data_table Block Kit example to the block-kit showcase, mirroring the official docs example.

The data_table block is net-new and is not yet in slack_sdk (confirmed absent in the pinned slack_sdk==3.42.0). This example defines a small, typed DataTableBlock that subclasses the SDK's slack_sdk.models.blocks.Block base class, following the same implementation pattern the SDK uses for TableBlock (validators for the required rows and caption fields, attributes covering rows, caption, page_size, row_header_column_index, block_id). When the SDK ships a native DataTableBlock, this example can be swapped to import it directly.

• block-kit/src/blocks/data_table.py — example01() returning a DataTableBlock (raw text header + raw text / rich text cells, with a caption), mirroring the docs example JSON.
• block-kit/tests/blocks/test_data_table.py — asserts the produced block deep-equals the expected literal (same style as test_table.py / test_task_card.py).
• block-kit/README.md — alphabetical "Data table" entry with docs + implementation links.

Docs

https://docs.slack.dev/reference/block-kit/blocks/data-table-block

Validation

Run in block-kit/ against the pinned requirements.txt (slack_sdk==3.42.0):

• ruff check — passed
• ruff format --diff --check — passed (new files already formatted)
• mypy src/blocks/data_table.py tests/blocks/test_data_table.py — passed (no issues)
• pytest tests/blocks/test_data_table.py -v — passed (1 passed)
• Full pytest -v — all data table tests pass

Note: a repo-wide mypy ./**/*.py currently reports pre-existing errors in src/blocks/carousel.py (unrelated work not part of this PR); the files added here are mypy-clean.

Co-Authored-By: Claude svc-devxp-claude@slack-corp.com