Update SNSEvent.Message Coding Keys

[hiimtmac]

Update SNSEvent.Message coding keys to match go models & documentation

Motivation:

When trying to decode an SNSEvent as the lambda's event (using runtime 1.0.0-alpha.1), I am getting:

2023-05-18T14:51:32+0000 warning Lambda : lifecycleIteration=16 lambda handler returned an error: requestDecoding(Swift.DecodingError.keyNotFound(CodingKeys(stringValue: "SigningCertURL", intValue: nil), Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "Records", intValue: nil), _JSONKey(stringValue: "Index 0", intValue: 0), CodingKeys(stringValue: "Sns", intValue: nil)], debugDescription: "No value associated with key CodingKeys(stringValue: \"SigningCertURL\", intValue: nil) (\"SigningCertURL\").", underlyingError: nil)))

It seems that the model's coding keys have changed with the proper keys being SigningCertUrl and UnsubscribeUrl (as opposed to SigningCertURL and UnsubscribeURL).

https://github.com/aws/aws-lambda-go/blob/771b391678d3f54bfa38531774d656f5e0f2ab58/events/sns.go#L28
https://github.com/aws/aws-lambda-go/blob/771b391678d3f54bfa38531774d656f5e0f2ab58/events/sns.go#L30
https://docs.aws.amazon.com/lambda/latest/dg/with-sns.html

Modifications:

Update the coding key raw values for SNSEvent.Message.signingCertURL and SNSEvent.Message.unsubscribeURL

Result:

Lamda handler can now successfully decode SNSEvent in then handle(_:context:) method.

[tomerd]

@swift-server-bot test this please

[tomerd]

thanks @hiimtmac