assume role chain parse tree (#632)

* assume role chain parse tree

This commit builds the parsing half of our AssumeRoleProvider implementation. Our implementation decouples
the (fairly complex) task of actually turning a profile file into a series of providers from the (fairly simple)
task if iterating through credential providers and executing them.

The descriptive test cases from the spec have been ported to JSON and some new ones have been added and they cover our implementation fairly exhaustively.

Future commits will implement a named-provider factory, wire up the STS client and actually turn this into a credentials provider.

* assume role chain parse tree

This commit builds the parsing half of our AssumeRoleProvider implementation. Our implementation decouples
the (fairly complex) task of actually turning a profile file into a series of providers from the (fairly simple)
task if iterating through credential providers and executing them.

The descriptive test cases from the spec have been ported to JSON and some new ones have been added and they cover our implementation fairly exhaustively.

Future commits will implement a named-provider factory, wire up the STS client and actually turn this into a credentials provider.

* Update aws/rust-runtime/aws-auth-providers/src/profile/repr.rs

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* Deal with the aws-auth-providers package being special

* add copyright header

* update changelog

Co-authored-by: John DiSanti <jdisanti@amazon.com>