Support description of mrkdwn type in OptionObject

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.

Start with com.slack.api.model.block.composition.OptionObject and inspect how its description is represented and deserialized. Verify the model behavior for option descriptions whose type is mrkdwn as well as plain_text, with completion shown by preserving both forms without lossy coercion.