add custom user agent suffix to AWS KMS MKP client

[mattsb42-aws]

Problem

We would like the AWS KMS client used in the default AWS KMS MKP to identify itself as originating from this CLI.

Solution

Like we do with the clients in Java and Python, we should append a special user agent string to the client's user agent.

Ref: https://github.com/awslabs/aws-encryption-sdk-python/blob/master/src/aws_encryption_sdk/key_providers/kms.py#L203-L206