Feature request: API to pass KMSMasterKeyProviderConfig to key providers' KMS clients

Currently, the best way to configure the KMS clients that Master Key Providers create, is to [subclass the desired provider](https://github.com/aws/aws-encryption-sdk-python/pull/440) to supply the desired KMS client configuration. This can be better handled by providing a dedicated API or argument, or to refactor the Python ESDK to use keyrings.

Context: https://github.com/aws/aws-encryption-sdk-python/issues/446

首先阅读 issue #446 中的上下文以及 pull request #440 中的 subclassing 方法。确定将 KMSMasterKeyProviderConfig 传递给 provider 创建的 KMS 客户端的预期 API，并以一种有文档说明且经过测试、无需 subclassing 即可配置这些客户端的方式作为完成标准。

由索引模型根据 Issue 内容生成。