[keyring] add is_discovery parameter to KMS keyring

[mattsb42-aws]

I originally had discovery defined as a derived property in the KMS keyring because that is how we currently have it defined in the spec. However, all of the other KMS keyring implementations have it as an explicit parameter, and after discussing it with the team we decided that was the correct UX. As such, we'll be updating the spec accordingly[1] and we need to update this KMS keyring before release.

[1] awslabs/aws-encryption-sdk-specification#84