Remove dependency on active_attr

[ianks]

Is your feature request related to a problem? Please describe.

Upon examining our dependencies, I noticed that sniffer pulls in a lot of dependencies that we do not use, and do not want to bring into our Hanami app.

    active_attr (0.15.1)
      actionpack (>= 3.0.2, < 6.2)
      activemodel (>= 3.0.2, < 6.2)
      activesupport (>= 3.0.2, < 6.2)

Describe the solution you'd like

Would it be possible to implement this gem without depending on the active* gems? To be more friendly to the non-rails ecosystem, something like dry-struct could be used, although not required.