Support sharp transformations with a new option - transformer

[ranihorev]

Added sharp transformer as a new option to allow image transformation. Passes all tests including a new test with that option.

Example:

const transformer = sharp().resize({ width: 100 });
var storage = multerS3({ s3: s3, bucket: 'test', transformer: transformer }) 

[LinusU]

Sorry for the late reply 🙈

I think that this is better addressed in Multer than in the individual storage engines, and with the release candidate of Multer 3.x this should be really easy!

Closing in favour of solving this upstream...