Allow specifying additional authorized_keys

[LBGarber]

Users are currently unable to provision Linode Instances with a predefined public or private key. If ssh_private_key_file is specified, the public key will always be set to nil and provisioning will fail.

This change allows users to set the authorized_keys field exposed by the Linode API, which allows for an Instance to be provisioned with any number of public keys.

For example:

source "linode" "example" {
  image             = "linode/alpine3.14"
  image_description = "My Alpine Image."
  image_label       = "packer-alpine"
  instance_label    = "temp-packer-alpine"
  instance_type     = "g6-nanode-1"
  linode_token      = var.linode_token
  region            = "us-east"

  authorized_keys = [chomp(file("/path/to/my/pubkey.pub"))]

  ssh_username      = "root"
  ssh_private_key_file = "/path/to/my/privkey"
}

I've also added a quick test to ensure the authorized_keys field is handled correctly.

Please let me know if you have any suggestions or more idiomatic solutions to this problem, especially if there is a preferred way to handle this with a communicator!

Thanks! 😄

Closes #18

[azr]

LGTM

[azr]

Removing this RootSSHKey field that was previously settable is technically a backwards incompatible change, but at the same time it's not documented nor used. So this looks okay to me.