feat: ssh: use secret in key_data

[idrista]

Support secret lookup for key_data

What’s changed?

• key_data can now be a string to reference a secret, just like registry.username and registry.password.
• To avoid breaking changes, the behavior is reversed compared to registry:
  • Registry.username | Registry.password : Array → lookup secret, String → raw value
  • SSH.key_data: Array → raw value, String → lookup secret

Why?

• This makes it easier to manage SSH keys securely using .kamal/secrets.
• Avoids exposing sensitive data in Git.

[lucashfreitas]

would be very nice!

[djmb]

Superseded by #1620