BlockKit Helper

Hello, I've spent a few hours putting together what I think is a fairly good prototype for composing BlockKit-based messages in Ruby. Currently, it has what I personally need from it for my own project, and no tests or meaningful docs. That's adequate for my personal project, but not for contributing to something bigger.

I'd like to gauge how much interest there would be in integrating my stuff into slack-ruby-client. (Alternatively, I could look into releasing it as a separate gem that can merely plug into slack-ruby-client.) From that, I figure I could decide how much work to put in to finish fleshing out all the other classes and writing an adequate test suite and documentation for the whole thing.

Current progress is over here: https://github.com/slack-ruby/slack-ruby-client/compare/master...saraid:saraid/slack-blockkit-helper

Start by reviewing the linked comparison from master to understand the BlockKit helper prototype and its current scope. The issue mentions no specific files or tests; determine whether the work belongs in slack-ruby-client or a separate gem, then define the remaining classes, test coverage, and documentation needed for a complete contribution.