Add expand attribute to SectionBlock

[henrinormak]

The API docs list the attribute expand, which is currently missing from the SDK version in SectionBlock class.

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

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[henrinormak]

I am willing to also contribute a PR with this - I might need some help with it (feedback via code review), but in general I think I understand how the model definition works.

[hello-ashleyintech]

@henrinormak Thank you for raising this, @henrinormak! If you are able to open a PR for this, I would be happy to give you a code review! If not, let me know and I can open a PR for this!