Add support for custom search_document_id with record loading

[khasinski]

When a model defines a custom search_document_id, searching with load: true would fail to find records because Searchkick used the custom ID to query the database primary key.

This adds support for models to define find_by_search_document_ids class method to handle the mapping from custom search document IDs back to database records.

Example:

  class Product < ApplicationRecord
    searchkick

    def search_document_id
      "tenant_#{tenant_id}_#{id}"
    end

    def self.find_by_search_document_ids(search_ids)
      ids = search_ids.map { |sid| sid.split("_").last }
      where(id: ids)
    end
  end

Fixes #1732