[Enh] Switch OpenNeuroPET Created JSON's with Schema

[bendhouseart]

PET2BIDS was developed before the BIDS Schema, but now that the schema is mature it's time to ditch our non-standardized versions of JSONs for something more stable.

See:

https://bids-specification.readthedocs.io/en/latest/schema.json

https://bids-specification.readthedocs.io/en/stable/schema.json

[CPernet]

should it be that inside /metadata we have import bidsschematools as bst and regenerate [PET_metadata.json}(https://github.com/openneuropet/PET2BIDS/blob/main/metadata/PET_metadata.json) this would avoid the started changes -- it is IMO a better option than matlab library feching stuff from the python library. Instead we simply have our json file updated as needed.

[CPernet]

https://jsr.io/@bids/schema