Add repr=False to attributes in EncryptionContext

[johnwalker]

https://github.com/aws/aws-dynamodb-encryption-python/blob/master/src/dynamodb_encryption_sdk/structures.py#L49-L78

We should add repr=False to the attributes section in the EncryptionContext, to make the string representation safe within material providers. This is useful particularly in the context of logging.

Similar to aws/aws-dynamodb-encryption-java#100