make participants.tsv optional

[yarikoptic]

ATM seems to be required according to https://github.com/datalad/datalad-registry/pull/261

  File "/usr/local/lib/python3.9/dist-packages/datalad_metalad/extract.py", line 461, in perform_dataset_metadata_extraction
    if extractor.get_required_content() is False:
  File "/usr/local/lib/python3.9/dist-packages/datalad_neuroimaging/extractors/bids_dataset.py", line 150, in get_required_content
    bids_dir = _find_bids_root(self.dataset.path)
  File "/usr/local/lib/python3.9/dist-packages/datalad_neuroimaging/extractors/bids_dataset.py", line 308, in _find_bids_root
    raise FileNotFoundError(msg)
FileNotFoundError: The file 'participants.tsv' should be part of the BIDS dataset in order for the 'bids_dataset' extractor to function correctly