Bounce Class Constants

[deelawn]

The goal of the below description is to define the date from the following package inside the events directory, perhaps in a file named bounce_class.go. It should contain the information defined here: https://www.sparkpost.com/docs/deliverability/bounce-classification-codes/?_ga=2.267685964.391106097.1594834092-1755179861.1592932304

It would be nice to have the bounce class codes defined as constants such as UndeterminedBounceClass="1", InvalidRecipientBounceClass="10", etc. It'd also be nice to have a type called BounceClassAttributes that is a struct with two fields, Description and Category. Category would be a newly defined type, BounceCategory. Then a map could be defined that has a key of the bounce class code and a value of type BounceClassAttributres.

I'm not sure if it would be wise to make BounceClass its own type, since users are currently using this as a string type and modifying this would break any existing code someone has written where they (for whatever reason) assign a string value to BounceClass where that string value is not a valid bounce class code.

This would be helpful to me, so I can implement it if this is something that would be accepted.