[BUG] channels and events files do not accept desc for derivative datastest

[neuromechanist]

Describe your problem in detail.

Created a BIDS derivative dataset with _desc to indicate what has been changed. The new BIDS validator throws an error for channels, events, and subsequently, electodes and coordsystem as they can't accept `desc'.

Example log:

/sub-NDARAC904DMU/eeg/sub-NDARAC904DMU_task-contrastChangeDetection_run-1_desc-dataElectrodes_channels.tsv (desc not in rule rules.files.raw.channels.channels)
/sub-NDARAC904DMU/eeg/sub-NDARAC904DMU_task-contrastChangeDetection_run-1_desc-dataElectrodes_events.tsv (desc not in rule rules.files.raw.task.events)
/sub-NDARAC904DMU/eeg/sub-NDARAC904DMU_task-contrastChangeDetection_run-2_desc-dataElectrodes_channels.tsv (desc not in rule rules.files.raw.channels.channels)
/sub-NDARAC904DMU/eeg/sub-NDARAC904DMU_task-contrastChangeDetection_run-2_desc-dataElectrodes_events.tsv (desc not in rule rules.files.raw.task.events)

I have already set "DatasetType": "derivative", and the lack of the same error for eeg files indicates that the problem is isolated to events and channels.

Describe what you expected.

All files should accept the desc entity as they all could be updated in a derivative dataset.

BIDS specification section

The reason seems to stem from the lack of explicit mention of these file types under src/schema/rules/files/deriv/preprocessed_data.yaml.

cc:@arnodelorme