Add repr=False to attributes in EncryptionContext

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 https://github.com/aws/aws-dynamodb-encryption-java/issues/100