Support creating users on login

`aws ec2-instance-connect send-ssh-public-key` (in the AWS CLI) lets me upload an SSH for users that do not exist. This makes sense and is good.
I would like to be able to have those users created on-login, though (according to a provided user template, perhaps).
Our use case is to allow developers to come and go from the team without needing to manually create accounts for them on our fleet of hundreds of instances to grant them the ability to login to those instances as required.

Start from the `aws ec2-instance-connect send-ssh-public-key` login flow and read this repository's SSH daemon configuration and EC2 instance scripting for the supported Linux distributions. Clarify the user-template format, creation timing, account properties, and security expectations before implementation; done should mean users absent from an instance can log in through the described flow and the behavior is validated across the relevant packaging configurations.

Written by the indexing model from the issue text.