Create a symlink file, which can be followed by get_file().
Unlike unix symlinks, symlink files can be created by this function are normal text files and can be
uploaded to object stores via ObjectStore.upload_object() or loggers via Logger.upload_file()
that otherwise would not support unix-style symlinks.
existing_path (str) – The name of existing object that the symlink file should point to.
destination_filename (str | Path) – The filename to which to write the symlink.
It must end in '.symlink'.