Add explicit attributes for fields in resources, and rbis for resources

[helenye-stripe]

Why?

This is the first step in #1479 - this adds explicit attributes for fields in resources, and adds RBIs with type annotations for them. This is not yet ready for use as it does not define any methods, so I have not included the RBIs in the gemspec.

What?

I recommend reviewing this mostly through the codegen PR, as this is almost entirely generated changes.

Important notes:

• There are a set of files that do not have any methods, and are returned solely in deprecated APIs. However, they are described in the return types for these methods, so I have added them and their corresponding RBIs. They are:
  • lib/stripe/resources/application.rb
  • lib/stripe/resources/billing/alert_triggered.rb
  • lib/stripe/resources/billing/meter_error_report.rb
  • lib/stripe/resources/connect_collection_transfer.rb
  • lib/stripe/resources/entitlements/active_entitlement_summary.rb
  • lib/stripe/resources/issuing/settlement.rb
  • lib/stripe/resources/quote_line.rb
  • lib/stripe/resources/reserve_transaction.rb
  • lib/stripe/resources/source_mandate_notification.rb
  • lib/stripe/resources/tax_deducted_at_source.rb
• Added lib/stripe/resources/amount.rb and rbi/stripe/resources/amount.rbi because it is a type used in V2.
• Added an additional step in StripeObject.add_accessors (lib/stripe/stripe_object.rb) to appropriately set instance variables upon creation.
• Updated .rubocop.yml to ignore variable numbers, since we have field names in various conventions in the API.