librarian: consider moving `tag_format` to config.yaml

This currently exists in `state.yaml` but really feels like it belongs in `config.yaml`. It's a configuration value that will not be updated by librarian itself.

Example config.yaml:

```yaml
# repository default
tag_format: "v{version}"

# per library configuration
libraries:
  - id: google-cloud-example
    tag_format: "{id}/v{version}"
```