Append an SSH public key to an existing instance's authorized_keys.

Behavior:

• If public_key is provided, that raw key material is appended to authorized_keys. If the same public key is already attached, the request is a no-op (no error).
• If public_key is omitted, Thunder generates a new key pair, appends the public half to authorized_keys, and returns the private key in the response. The private key is only returned once — store it immediately.

Note: SSH keys saved in your account (Authentication section of the console) are a separate store and are NOT automatically attached to instances. To use one of those keys, copy its public key material into the public_key field on this request.