feat: Add method to GtfsFeedLoader to access GtfsTableDescriptors.

[bdferris-v2]

As part of work on #1564, expose the set of GtfsTableDescriptors in GtfsFeedLoader so that their properties might be modified at runtime.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[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: 7c855d8
Download the full acceptance test report here (report will disappear after 90 days).
✅ Rule acceptance tests passed.

[github-actions]

✅ Rule acceptance tests passed.
New Errors: 0 out of 1478 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Errors: 0 out of 1478 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
New Warnings: 0 out of 1478 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Warnings: 0 out of 1478 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
0 out of 1478 sources (~0 %) are corrupted.
Commit: 4a35472
Download the full acceptance test report here (report will disappear after 90 days).
✅ Rule acceptance tests passed.