KMS Multi-Region Key aware keyring (AWS SDK for C++)

Data Structures
class	Aws::Cryptosdk::KmsMrkAwareSymmetricKeyring::Builder
class	Aws::Cryptosdk::KmsMrkAwareSymmetricKeyring::MultiKeyringBuilder

Detailed Description

We have implemented a keyring on top of KMS, which uses the AWS SDK for C++ as its underlying KMS client.

Because there is no pure-C AWS KMS client at the moment, C++ is required to use this keyring. We expect that a parallel pure-C API will be added in the future when a pure-C KMS client becomes available.