Make slash commands defined in extensions return `SlashCommandOutput` (#13237)

This PR extends the interface for slash commands defined in extensions
to have them return `SlashCommandOutput`.

This allows for slash commands to return multiple output sections for a
single piece of generated text.

Note that we don't allow specifying the icon to display in the
placeholder, as we don't want to commit to that in our API at the
moment.

Release Notes:

- N/A