feat(block-kit): add block element examples

[zimeg]

Summary

Adds dedicated block element examples under block-kit/, one file per element in src/block_elements/ with a matching test in tests/block_elements/.

Each example mirrors the JSON payload(s) from the element's docs.slack.dev reference page verbatim — hosted in a valid block (or input) where the element requires one, so each is a runnable payload rather than a bare element that can't be sent on its own. Where a reference page documents multiple payloads, every one is exemplified, and each test asserts the complete reference JSON.

This PR is the home for block elements the SDK implements; elements not yet implemented in the SDK are a separate follow-up.

Elements covered

Element	Examples
Button	4 (regular, primary, link, hosted section + actions)
Checkboxes	1
Date picker	1
Datetime picker	1
Email input	1
File input	1
Image	3 (image_url, slack_file by url, slack_file by id)
Multi-select menu	5 (static, external, users, conversations, channels)
Number input	1
Overflow menu	1
Plain-text input	1
Radio button group	1
Select menu	5 (static, external, users, conversations, channels)
Time picker	1
URL input	1
Workflow button	1

Deferred

The button reference page also documents an agent_prompt variant (Slackbot hand-off), which the SDK does not yet implement — deferred here rather than shipping an example whose serialized payload wouldn't include the field.

Testing

ruff check, ruff format --check, mypy, and pytest all pass in block-kit/ (63 tests).