[ENH] Include RawSources in sidecar JSON files

[adam2392]

Describe the problem

In bids-standard/bids-specification#906, I am including RawSources into sidecar JSON files. These will link to the original raw.filenames.

Describe your solution

Update write_raw_bids to allow this functionality. Make the writing default behavior, unless anonymization is desired. In the anonymization dictionary, we can allow the writing of RawSources to be turned off.

Describe possible alternatives

n/a