Allow client_id configuration on instance config (#952)

It's necessary to allow `client_id` to be configured in order to move
towards supporting `StripeClient` instances.

lib/stripe.rb

@@ -79,13 +79,12 @@ class << self
     def_delegators :@configuration, :logger, :logger=
     def_delegators :@configuration, :max_network_retries, :max_network_retries=
     def_delegators :@configuration, :enable_telemetry=, :enable_telemetry?
+    def_delegators :@configuration, :client_id=, :client_id
 
     # Internal configurations
     def_delegators :@configuration, :max_network_retry_delay
     def_delegators :@configuration, :initial_network_retry_delay
     def_delegators :@configuration, :ca_store
-
-    attr_accessor :client_id
   end
 
   # Gets the application for a plugin that's identified some. See