add "ssh-key get --project-id $ID"

[displague]

This PR add support for listing ssh-keys in a given project.

metal ssh-keys get -P 

[displague]

Previous description:

This PR, as drafted, introduces a bug because the project-id from ~/.config/equinix/metal.yaml will always be defined.
A user will therefor not be able to get user ssh-keys, they can only get project ssh keys.

Stated another way, project id is defined by viper from config args. This is preventing the ssh-key get from defining project id.

Possible solutions:

• use a different variable name for project selection (metal ssh-key get --project --project-id {id})
• use a different action name for user or project ssh key listing (metal project ssh-keys, metal ssh-key project-get)
• default to project listing and only attempt user ssh-key listing in the presence of a new argument (metal ssh-key get --user)
• find a way to ignore the project-id defined by viper (metal ssh-key get --project-id $id (ignoring the viper defined project-id))

---

I went with the first option, but expanded --project to --project-ssh-keys (with short flag -P):
metal ssh-key get --project-ssh-keys --project-id {id}

Since metal init sets a default project, metal ssh-keys get -P will list project ssh keys for the configured project.