Don't print AttributeValues in EncryptionContext.toString()

[johnwalker]

https://github.com/aws/aws-dynamodb-encryption-java/blob/master/sdk1/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/encryption/EncryptionContext.java#L220

https://github.com/aws/aws-dynamodb-encryption-java/blob/master/sdk2/src/main/java/software/amazon/cryptools/dynamodbencryptionclientsdk2/encryption/EncryptionContext.java#L182

A safe toString() method for EncryptionContext would be extremely useful for logging within material providers, and we can get there by excluding attributeValues from being printed. Since it is likely that tests rely on this today, we should consider doing this with a minor bump. (Note: We haven't published the sdk2 version yet, so it's not necessary to do a bump there.)