[Bug]: missing rest_api_key Configuration accessor

[alexrecital]

What happened?

Hi there, bumping the gem version from 2.2.1 to 5.3.0 it looks like there is an accessor issue for the configuration:

Loading development environment (Rails 8.1.3)
recital-backend(dev):001> require "onesignal"
recital-backend(dev):002>   OneSignal.configure { |c| c.rest_api_key = "key" }
recital-backend(dev):003>   # => NoMethodError: undefined method 'rest_api_key=' for an instance of OneSignal::Configuration
(recital-backend):2:in 'block in <main>': undefined method 'rest_api_key=' for an instance of OneSignal::Configuration (NoMethodError)
	from (recital-backend):2:in '<main>'

Steps to reproduce?

1. Using rails 8.1.3, add the onesignal (5.3.0) gem
2. Start the rails console
3. Attempt to configure the `rest_api_key` per the current doc guidelines

What did you expect to happen?

A successful initialization

Relevant log output

Loading development environment (Rails 8.1.3)
recital-backend(dev):001> require "onesignal"
recital-backend(dev):002>   OneSignal.configure { |c| c.rest_api_key = "key" }
recital-backend(dev):003>   # => NoMethodError: undefined method 'rest_api_key=' for an instance of OneSignal::Configuration
(recital-backend):2:in 'block in <main>': undefined method 'rest_api_key=' for an instance of OneSignal::Configuration (NoMethodError)
	from (recital-backend):2:in '<main>'

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