Add an equivalent of river.WorkFunc for Ruby

[brandur]

Just noticed while doing a manual review on docs changes that Ruby
doesn't have an equivalent to Go's river.WorkFunc whereas the rest of
the languages do.

Here, let workers.add take a kind and closure:

workers.add(:send_message) do |job|
  puts "Message: #{job.args.fetch("message")}"
end

Insertions would have to come from another language or use JobArgsHash:

client.insert River::JobArgsHash.new(:send_message, message: "Hello!")

This admittedly isn't a huge improvement, but I think it's probably
worth doing just because both Rust and TypeScript have something
extremely similar. This puts all languages on equal footing.

Here, add one in. It's arguable as to whether it adds all that much
compared to a normal class which is already pretty terse, but it does
let you

[brandur]

@bgentry This one's not a big deal, but just happened to notice it while manual inspecting docs.

[bgentry]

Huh, not sure if this can some how be conformance tested but it'd be nice to ensure we have comparable feature sets across the ports.

[brandur]

thx!

Huh, not sure if this can some how be conformance tested but it'd be nice to ensure we have comparable feature sets across the ports.

Yeah, maybe something we can figure out. That said, I think once we get a couple passes in to pick up any of these omissions, they're pretty unlikely to regress.