previews: add a preview subsection to librarian.yaml Library entry config

[noahdietz]

Preview clients will be configured via their Stable client entry. A preview subsection of type Library is added to Library schema that contains preview-specific overrides. Example:

libraries:
  - name: foo
    version: 1.44.0
    apis:
      - path: google/cloud/foo/v1
    go:
      opt_args_by_api:
        google/cloud/foo/v1:
          - transport=rest
    preview: # inherits from stable parent
      version: 1.45.0-preview.1
      go: # Go config overrides for preview