iliana's neat ca thing but serverless

Notes

1. You probably shouldn't use this, I just wanted to see what running a lambda container was like.

2. If you do use this, configure your lambda IAM role so that it has permissions to do kms::sign things, and make sure you set the environment variable SSHCA_KEY_ID to be the key id you wish to sign with.

3. If you do deploy this, slap cognito or something in front of it. Idk, I don't know how security works.

Events should look like this:

{
    "keydata":"ssh-ed55519 AAAAAAAAAAAAAAAAAAAAAAAAAA comment",
    "user": "myusername"
}

At some point, I'll make it automatically guess 'user' based on the cognito context.