Do not require `application package` or `application_root` to initialize `VespaCloud`

[thomasht86]

Currently:

  # 1. Initialize VespaCloud with an application package and existing API key for control plane access.
  vespa_cloud = VespaCloud(
      tenant="my-tenant",
      application="my-application",
      application_package=app_package,
      key_location="/path/to/private-key.pem",
  )

  # 2. Initialize VespaCloud from disk folder by interactive control plane auth.
  vespa_cloud = VespaCloud(
      tenant="my-tenant",
      application="my-application",
      application_root="/path/to/application",
  )

  # 3. Initialize VespaCloud with an application package and token-based data plane access.
  vespa_cloud = VespaCloud(
      tenant="my-tenant",
      application="my-application",
      application_package=app_package,
      auth_client_token_id="my-token-id", # Must be added in Vespa Cloud Console
  )

But there are cases where one just needs a control plane connection to an existing application, and only use delete(), get_schema or other methods that should not require providing the package.