implementation: re-designing the AWS KMS keyring

Following up from https://github.com/awslabs/aws-encryption-sdk-specification/pull/148 , we should update the AWS KMS keyring the match the proposal.

This proposal affects/resolves the following issues: #139, #94, #90, #84, #59, #16, #15, #49, #83, #40

Please see [this change doc](https://github.com/awslabs/aws-encryption-sdk-specification/blob/master/proposals/2020-07-01_aws-kms-keyring-redesign/change.md) for more information.

* Python: https://github.com/aws/aws-encryption-sdk-python/issues/276
* JavaScript: https://github.com/aws/aws-encryption-sdk-javascript/issues/380
* C: https://github.com/aws/aws-encryption-sdk-c/issues/555
* Java: https://github.com/aws/aws-encryption-sdk-java/issues/199