[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.

[OppOops]

Here is the suggestion to use normal git commands like this, which will need to change the ssh-key file permission and add the known_hosts in to list.
Below we suppose the git host is github.com :

skip_clone: true
steps:
  - name: clone
    image: alpine/git
    commands:
      #if the content is base64, just decode it with base64 -d ... > pull_key
      - echo "$CLONE_SSH_KEY_CONTENT" > pull_key
      - chmod 600 pull_key
      - mkdir -p /root/.ssh
      - ssh-keyscan github.com >> /root/.ssh/known_hosts
      
      # below commands simiulate the woodpeckerci/pulgin-git
      - git init
      - git remote add origin $CI_REPO_CLONE_SSH_URL
      - git config core.sshCommand 'ssh -i pull_key'
      - git fetch && git checkout $CI_COMMIT_BRANCH
    secrets: [ CLONE_SSH_KEY_CONTENT ]

pull_key will leave in the default workspace. If we use woodpeckerci/plugin-git to fetch the repo, the ~/.ssh/known_hosts needs to be add further. Now I use the clone like the following:

skip_clone: true
steps:
  - name: copy-key
    image: alpine/git
    commands:
      #if the content is base64, just decode it with base64 -d ... > pull_key
      - echo "$CLONE_SSH_KEY_CONTENT" > pull_key
      - chmod 600 pull_key
      - mkdir -p /root/.ssh
      - ssh-keyscan github.com >> /root/.ssh/known_hosts
    secrets: [ CLONE_SSH_KEY_CONTENT ]

  - name: clone
    image: woodpeckerci/plugin-git
    settings:
      - use-ssh : true
      - ssh-key : pull_key

[ser]

It's very inconvenient to use a special alpine image just to make such a simple operation, and it makes the woodpecker yaml file unnecessarily bloated...