Support for asymmetric cryptographic operations

Now that KMS [supports asymmetric keys](https://aws.amazon.com/blogs/security/digital-signing-asymmetric-keys-aws-kms/), it suffers from the same dilemma as symmetric operations: once you [sign](https://docs.aws.amazon.com/kms/latest/APIReference/API_Sign.html), you have to pass not only the signature but key id and signature type to the [verify](https://docs.aws.amazon.com/kms/latest/APIReference/API_Verify.html) API, leaving KMS users to work out how to store and pass along this metadata. The Encyption SDK has very effectively solved this problem for symmetric operations, and it would be great to have it extended to cover asymmetric operations (primarily signing, but potentially encryption too).

Start by reviewing how the Encryption SDK handles metadata for symmetric operations, then compare the AWS KMS Sign and Verify APIs linked in the issue. The issue does not name files, tests, or an entry point, and its scope is unresolved between signing and encryption; completion would require defining the asymmetric-operation design and its acceptance criteria.

Written by the indexing model from the issue text.