feat: Make GtfsTableDescriptor.required a mutable property

[bdferris-v2]

Per discussion in #1564, I'm looking to add base API support for changing the "required" status of some GTFS files/tables at run-time, as opposed to compile time. This PR achieves that by making "required" a mutable field in GtfsTableDescriptor to support dynamic configuration. Additionally, this PR consolidates some duplicate "isRequired" methods from GtfsTableContainer to make GtfsTableDescriptor the sole source of this info.

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[github-actions]

✅ Rule acceptance tests passed.
New Errors: 0 out of 1470 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Errors: 0 out of 1470 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
New Warnings: 0 out of 1470 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Warnings: 0 out of 1470 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
0 out of 1470 sources (~0 %) are corrupted.
Commit: 707890d
Download the full acceptance test report here (report will disappear after 90 days).
✅ Rule acceptance tests passed.

[jcpitre]

LGTM

[github-actions]

✅ Rule acceptance tests passed.
New Errors: 0 out of 1470 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Errors: 0 out of 1470 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
New Warnings: 0 out of 1470 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Warnings: 0 out of 1470 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
0 out of 1470 sources (~0 %) are corrupted.
Commit: 1dca978
Download the full acceptance test report here (report will disappear after 90 days).
✅ Rule acceptance tests passed.