feat(block-kit): add composition object examples

[zimeg]

Summary

Adds example projects for the Block Kit composition objects documented at docs.slack.dev. Composition objects never appear on their own, so each example reproduces the object's full reference payload hosted inside a valid block, element, or view — exactly as the docs page shows it. Every example ships a test asserting the complete serialized JSON.

Mirrors the conventions of the block element examples PR (#227): one exampleNN() per docs reference payload under src/composition_objects/, matched by a test under tests/composition_objects/ comparing to_dict() against the complete expected object.

Coverage

Composition object	Examples	Notes
Text	1	mrkdwn text hosted in a section
Option	2	the single option object, and options in a select menu
Option group	1	option groups in a select menu
Confirmation dialog	1	a button's confirm dialog
Dispatch action configuration	1	on a multiline plain-text input
Conversation filter	1	a conversations select filter in a modal view
Slack file	2	image block by file URL, and by file ID
Trigger	1	a workflow button trigger
Workflow	1	a workflow button workflow

slack_icon is documented but has no standalone reference payload (it is a sub-object of the card block, which the SDK does not yet expose), so it is not exemplified here.

Checks

Run from block-kit/, matching the pytest@python3.14 CI job:

• ruff check — passed
• ruff format --diff --check — passed (57 files)
• mypy . — passed (56 source files)
• pytest — 45 passed