Support json flag in heroku certs

Support --json flag in heroku certs command following pattern in other methods.
https://github.com/heroku/cli/blob/v10.12.0/packages/cli/src/commands/certs/index.ts

This would make it much easier to automate ssl updates when you need to know the name of the cert to perform an update.
I was unable to find an existing issue on the topic nor any discussion on why most of the command don't support json output.