Get the channel-specific webhook URLs for a bot the current user can manage. A bot has one URL per channel it can access. POST message content to a returned webhookUrl and authenticate with a token from IssueBotCredential or CreateBot in the returned credentialHeader; also send credentialScope in credentialScopeHeader. If no URLs are returned, add the bot to a channel with ManageBotChannelAccess or recreate it with CreateBot and channelId.