datasette publish lambda plugin

Refs #217 - create a publish plugin that can deploy to AWS Lambda.

https://docs.aws.amazon.com/lambda/latest/dg/limits.html says lambda packages can be up to 50 MB, so this would only work with smaller databases (the command can check the filesize before attempting to package and deploy it).

Lambdas do get a 512 MB /tmp directory too, so for larger databases the function could start and then download up to 512MB from an S3 bucket - so the plugin could take an optional S3 bucket to write to and know how to upload the .db file there and then have the lambda download it on startup.