Add expand property to SectionBlock class

[ESteanes]

There is now an expand option in the Section Block API https://api.slack.com/reference/block-kit/blocks#section which isn't exposed in the Java SDK. If it was exposed, we would be able to control whether the content will me automatically expanded or not rather than Slack deciding for us.

expand	Boolean	Whether or not this section block's text should always expand when rendered. If false or not provided, it may be rendered with a 'see more' option to expand and show the full text. For AI Assistant apps, this allows the app to post long messages without users needing to click 'see more' to expand the message.	No

expand
Boolean
Whether or not this section block's text should always expand when rendered. If false or not provided, it may be rendered with a 'see more' option to expand and show the full text. For AI Assistant apps, this allows the app to post long messages without users needing to click 'see more' to expand the message.
No

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)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please make sure if this topic is specific to this SDK. For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. 🙇

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