feat: 1776 add flex features to list

[qcdyx]

Summary:

Closes #1776

Expected behavior: I discovered that the primary reason most of the stop_times.txt files in flex feeds are unparsable is the absence of the required stop_id field. So to test flex features, I commented @Required annotation in the GtfsStopTimesSchema and generated the following report. We need to discuss the logic behind the requirement of the stop_id field for non-flex feeds vs. flex feeds. (I cannot find a flex feed that meets Deviated Fixed Route criteria)

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