added AWS S3 Event Source

[jongreenwood-dunelm]

Until now there is no defined AWS S3 Event Source and the event defaults to LAMBDA EDGE.

This PR adds AWS_S3 as an EventSource type and exports the request response mapper.

It allows invocation like this

`` `
const handler = serverlessExpress({
app: service(),
eventSourceRoutes: {
AWS_S3: '/s3',
},
eventSource: {
getRequest: getRequestValuesFromS3,
getResponse: getResponseToS3,
},
eventSourceName: 'AWS_S3',
logSettings: {
level: 'debug',
},
})

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

[brettstack]

Hi @jongreenwood-dunelm I'd love to merge this but it's missing tests. Would you be able to add?