Feature request: Deploy Lambdas with concurrency

### Describe your idea/feature/enhancement

We have 100s of lambdas. We build outside of SAM using esbuild into single files. When we deploy with SAM it zips up the code, checks the checksum against S3 and then decides whether to update the Lambda or not. This is a pretty slow process for us, and takes ~15m.

### Proposal

SAM will zip + checksum + check s3 for each lambda in parallel batches, rather than 1 by 1.