DefaultCryptoMaterialsManager should raise an error if signer key is present in encryption context

Because `DefaultCryptoMaterialsManager` adds a value to the encryption context if a signed algorithm suite is used, we need to throw an error if the requested encryption context already contains that key.

https://github.com/aws/aws-encryption-sdk-java/blob/eb8702e82759cca53eddc3b5f8423e74ec31aff1/src/main/java/com/amazonaws/encryptionsdk/DefaultCryptoMaterialsManager.java#L55-L58

首先在 Python 仓库中定位 DefaultCryptoMaterialsManager，并将其对已签名算法加密上下文的处理与 DefaultCryptoMaterialsManager.java#L55-L58 中引用的 Java 实现进行比较。确认对于已经包含签名者密钥条目的上下文，预期行为是什么，然后添加或更新覆盖范围，使该操作在这种情况下引发错误。

由索引模型根据 Issue 内容生成。