Adding toString method to the S3EventNotification.

[msailes]

Issue #, if available:
#301

Description of changes:
Added default IntelliJ toString methods for all attributes in all classes. This is quite a nested object so the toString output is quite large. It doesn't contain the S3 object itself though.

Example:

{
   [
      S3EventNotificationRecord{
         "awsRegion=""eu-central-1",
         "eventName=""ObjectCreated:Put",
         "eventSource=""aws:s3",
         "eventTime=1970-01-01T00":"00":00.000Z,
         "eventVersion=""2.0",
         "requestParameters=RequestParametersEntity"{
            "sourceIPAddress=""127.0.0.1"
         },
         "responseElements=ResponseElementsEntity"{
            xAmzId2="EXAMPLE123/5678abcdefghijklambdaisawesome/mnopqrstuvwxyzABCDEFGH",
            "xAmzRequestId=""EXAMPLE123456789"
         },
         s3=S3Entity{
            "configurationId=""testConfigRule",
            bucket=S3BucketEntity{
               "name=""example-bucket",
               "ownerIdentity=UserIdentityEntity"{
                  "principalId=""EXAMPLE"
               },
               "arn=""arn:aws:s3:::example-bucket"
            },
            object=S3ObjectEntity{
               "key=""test/key",
               size=1024,
               "eTag=""0123456789abcdef0123456789abcdef",
               "versionId=""",
               "sequencer=""0A1B2C3D4E5F678901"
            },
            s3SchemaVersion="1.0"
         },
         "userIdentity=UserIdentityEntity"{
            "principalId=""EXAMPLE"
         }
      }
   ]
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[carlzogh]

LGTM - thanks!

[msailes]

No longer required, the functionality will existing post v4.