[cli] Allow to refer to local files in secrets.yaml and in instance.yaml using <file:xxx> syntax

[eolivelli]

Summary:

• in secrets.yaml and instance.yaml you can now refer to local files using the file:xxx syntax
• the file is automatically read from the local file system and copied into the YAML file
• text files are kept as they are
• binary files (for instance secure bundle zip files) are base64 encoded and prepended with the "base64:" prefix

Sample usage:

secrets:
  - name: vertex-ai
    id: vertex-ai
    data:
      url: https://us-central1-aiplatform.googleapis.com
      token: xxx
      serviceAccountJson: "<file:service-account.json>"
      region: us-central1
      project: myproject
  - name: astra
    id: astra
    data:
      username: xxx
      password: xxx
      secureBundle: "<file:secure-connect-bundle.zip>"

Please note that this is a CLI only feature, we will have to implement support for this syntax in the VS Code extension independently