[Feature] Add new properties for raw values

[EdmonDantes]

Please add properties ssh-key-raw to provide Base64 string of SSH key (not path to file) and custom-cert-raw to provide Base64 string of custom server certificate.

I tried to use this plugin in a Woodpecker with a Kubernetes backend. I can't add Docker volume to clone step, and I can't execute any command (for example to create a file). So I can not create any file and only set values to properties. I think it will be great if a user can provide a secret's values directly to properties.