convert all attrs classes to use custom init methods to enable mypy to work

[mattsb42-aws]

Issue #, if available: #4

Description of changes:
Because mypy cannot currently correctly read the init profile of an attrs-generated class, in order for mypy to work correctly we need to build out these __init__ shims. These enable mypy integration at the cost of making the codebase a bit messier. Once mypy and attrs can play well together we'll be removing these shims, so they should do literally nothing except:

1. take in parameters
2. set any default values
3. assign those parameters to instance attributes
4. call attrs to run all the validators
5. if present in the class, call the __attrs_post_init__ function (this function is used with attrs-generated classes for any custom logic that needs to happen at init time)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.