Define callbacks mode as a lambda

[stengineering0]

Is your feature request related to a problem? Please describe.
Currently the suggested way to perform the reindexing conditionally is

class Product < ApplicationRecord
  searchkick callbacks: false

  # add the callbacks manually
  after_commit :reindex, if: -> (model) { model.previous_changes.key?("name") } # use your own condition
end

The problem is that this approach is likely about the copy/paste action from Searchkick internals into the host application. The client is forced to investigate the sources of Searchkick, understand in details how and why it uses after_commit callback, paste the same into own application, apply the customizations, and as it always happen with code duplication - keep tracking the upstream sources in order to catch & reflect the breaking changes.

For instance, the suggested conditional reindexing approach from above relies on :inline callbacks mode under the hood. If a client normally uses :async and just wants to skip reindexing sometimes - it have to do something like:

class Product < ApplicationRecord
  searchkick callbacks: false

  # add the callbacks manually
  after_commit(if: -> (model) { model.previous_changes.key?("name") } { reindex(mode: :async) }
end

And still some important parts or Searchkick functionality might be broken now (or will be broken later after Searchkick upgrade) - for instance the global callbacks mode. This way the full-weight suggestion turns into:

class Product < ApplicationRecord
  searchkick callbacks: false

  # add the callbacks manually
  after_commit(if: -> (model) { Searchkick.callbacks? && model.previous_changes.key?("name") } { reindex(mode: :async) }
end

And still it does not fully respect the global callbacks mode such as Searchkick.callbacks(:bulk) do; end..

Describe the solution you'd like
Let's introduce the possibility to define callbacks mode as lambda for the model as follows:

class Product < ApplicationRecord
  searchkick callbacks: ->{ searchkick_callbacks_mode }

  def searchkick_callbacks_mode
    return false unless previous_changes.key?("name")
    return :inline if urgent?

    :async
  end
end

It will be evaluated within generic after_commit Searchkick callback.

I'm ready to provide PR once the feature will be accepted in general by code owners )