text: Title: How do I subscribe to block_action using bolt?
  Body: How do I subscribe to `block_action` using bolt? 
  
  I've found the decorator in the source code, but there are no comprehensive examples and the docs omit it. The decorator takes constrains argument, what do I pass there? I tried "button" and the value that I use for my action blocks, but nothing changes. I am just frustrated at this point.
  
  What I am trying to do:
  I am trying to open a modal when user clicks on a button in an ephemeral message.
  