Add missing OMOP tables to the default config

[kojikojikojikoji]

Part of #1294.

The default OMOP config did not define observation_period, visit_detail, observation or device_exposure, so loading them raised Table ... not found in config, though the docstring lists the last two. I used columns that exist in both CDM v5.3 and v5.4.

Added small CSVs to test-resources/omop and a test in tests/core/test_omop.py that loads all four.

from pyhealth.datasets import OMOPDataset

if __name__ == "__main__":
    tables = ["observation_period", "visit_detail", "observation", "device_exposure"]
    ds = OMOPDataset(root="test-resources/omop", tables=tables)
    patient = ds.get_patient("-4873075614181207858")
    for t in tables:
        print(t, len(patient.get_events(event_type=t)))