Add support for cloud provider CLIs

[cezmunsta]

Instead of using the ssh command, the cloud provider CLI tool may need to be used, e.g.

$ az ssh vm ...
$ gcloud compute ssh ...
$ doctl compute ssh ...

There is also the possibility that instead of executing a different command, the cloud provider's tool can be used to generate an SSH-compatible config. This would then only require the ability to support options that such files require.

• az: https://learn.microsoft.com/en-us/cli/azure/ssh?view=azure-cli-latest#az-ssh-config
• gcloud: https://cloud.google.com/sdk/gcloud/reference/compute/config-ssh

The simplest approach is most likely to be:

• Storing a flag that indicates which type of provider the entry relates to
• When the flag is ssh proceed with current processing
• When the flag is not ssh then a full set of arguments are simply stored in a single field in JSON format and then passed directly to the CLI tool
  This would then support any cloud provider tool, regardless of whether it supported generation of configs, etc