feat(block-kit): add block element examples

[zimeg]

Adds dedicated block-element examples under the block-kit project, mirroring the reference payloads shown on the Block Kit block-elements reference pages on docs.slack.dev. Each documented reference payload becomes one exampleNN(), and every example ships a test asserting the complete serialized JSON (compared as parsed JSON trees, so key order does not matter), following the existing blocks/ convention.

This is the home for all implemented block elements; anything not yet covered is left as follow-up.

Elements added

New package elements under block-kit/src/main/java (+ matching tests). The package is named elements (not blockelements) so the layout reads blocks/ + elements/, matching the docs.slack.dev path structure where Blocks and Block elements are siblings under Block Kit — and matching the sibling bolt-js-examples and bolt-python-examples PRs. The SDK's own com.slack.api.model.block.element.BlockElements import is unaffected by the rename.

18 elements, 31 examples:

• Button (4 examples): regular, primary, link, and buttons hosted in a section accessory + actions block
• Select menu (5 examples): static, external, users, conversations, channels
• Multi-select menu (5 examples): static, external, users, conversations, channels
• Radio button group (1)
• Date picker (1)
• Time picker (1)
• Datetime picker (1)
• Email input (1)
• URL input (1)
• Number input (1)
• File input (1)
• Feedback buttons (1)
• Icon button (1)
• Overflow menu (1)
• Plain-text input (1)
• Rich text input (1)
• Image (3 examples): image_url, slack_file by url, slack_file by id
• Workflow button (1)

The block-kit/README.md gains a "Block elements" section listing each element with its docs link, tagline, and a link to the implementation.

Notes on hosting

Elements are shown inside their host block exactly as the reference pages present them: selects, radio buttons, overflow, image, and workflow button as a section accessory; date/time/datetime/email/url/number/file/plain-text inputs inside an input block. Where the docs present an element inside a modal view, the element is hosted in its natural block so the test can assert the full block payload.

Deferred (SDK gaps)

• Checkboxes — the reference payload includes an option description typed as mrkdwn. The current model serializes an option description as plain_text, so the reference payload cannot be reproduced faithfully. Deferred until the description can carry a markdown text object.
• Button agent_prompt variant — not represented in the current button model, so it is omitted from the button examples.

Testing

mvn spotless:check and mvn test both pass for the block-kit project (all example tests green).