define decryption contracts for all keyrings

[mattsb42-aws]

This is a tracking issue for sub-issues for each keyring

• single-CMK AWS KMS keyring define decryption contract for single-CMK AWS KMS keyring #189
• AWS KMS discovery keyring define decryption contract for AWS KMS discovery keyring #190
• raw AES keyring define decryption contract for raw AES keyring #187
• raw RSA keyring define decryption contract for raw RSA keyring #188
• multi-keyring define decryption contract for multi-keyring #12