Add Puma plugin

[zachasme]

Possible solution for #3.

This adds a Puma plugin, in the same way Solid Queue provides one. Enabled using:

# in config/puma.rb
plugin :litestream if rails_env == "production"

which makes it simple to deploy with Kamal for example

[fractaledmind]

@zachasme I really like this! To get this ready for release I think we need a few more things:

• [ ] Fix the linter issues
• [ ] Add some tests (e.g. https://github.com/rails/solid_queue/blob/main/test/integration/puma/plugin_test.rb)
• [ ] Add a general section to the README (e.g. https://github.com/rails/solid_queue?tab=readme-ov-file#puma-plugin)
• [ ] Add a specific section on how to use this with a Dockerized app

One question after studying the Solid Queue implementation is whether or not the monitor_puma background thread is important. Do you have an opinion on that? My gut instinct it to copy their implementation fully, but I'm open to additional thoughts.

[zachasme]

Great! I'll fix the linter issues and add README section. And I think we should go with your instinct to copy the full implementation (the initial commit was just the bit I extracted to make it work on my own projects).

I'll have to think about how to make an integration test for the plugin, though.

[fractaledmind]

@zachasme Please rebase main to get the CI fix