Will Selby

### Story https://github.com/mettle/sendportal/issues/136 ### Description - Move SES quota check to message dispatch

Add Enum constants to resource classes

2

Would it be possible to have the relevant enum values added as constants to the resource classes please? Similar to what Stripe has; https://github.com/stripe/stripe-php/blob/0aa6a385e66ffe7e095372737d8afe491a94e015/lib/Invoice.php#L130 This saves us having to do...