Support description of mrkdwn type in OptionObject #1471

Need to read options in a checkbox from Slack using the Java SDK. Currently, description text objects inside an option are always coerced into a PlainTextObject even if their type property is set to mrkdwn.

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

Slack supports options with description text objects that use mrkdwn type. The SDK coerces them into a PlainTextObject, resulting in a lossy translation.

Consider changing the description field in the com.slack.api.model.block.composition.OptionObject to a com.slack.api.model.block.composition.TextObject so it can support either plain text or markdown.