Support keyword arguments in iteration jobs

[st0012]

It's now more common to have jobs that take keyword arguments, like MyJob.perform_later(foo: "bar"). But under the current implementation, build_enumerator can only take them as a hash argument instead of keyword arguments:

def build_enumerator(params, cursor:)
  foo = params[:foo]
  # ...
end

This commit improves that by allowing keyword arguments to be passed to build_enumerator and each_iteration as:

def build_enumerator(foo:, cursor:)
  # ...
end

[Mangara]

@st0012 Can you explain why you closed this? Does it not work? Or was it just unnecessary? Just looking to get some context if someone asks about this in the future.

[st0012]

I closed this because I later realised that the Shopify apps we care about actually can't adopt the keyword arguments pattern directly. And while this support won't negatively impact those apps either, having it would require job-iteration dropping Ruby 2.6 & 2.7 support at the same time.
So instead of dropping Ruby support for a feature nobody is asking for, I decided to close it and let others to pick it up if needed.