feat: metrics using single feature truth

[Alessandro100]

closes #820
Summary:

Updates the feature support to 6.0 for the gtfs metrics. Also consolidates the Features data to a source of truth in the frontend. Added sub group feature to metrics to accommodate Fares V2 and Flex

Expected behavior:

Metrics in the features should have no empty groups and should be displayed correctly

Testing tips:

Go to the GTFS metrics page (Feeds / Features) and play around with to make sure the features are displaying correctly

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

• Run the unit tests with ./scripts/api-tests.sh to make sure you didn't break anything
• Add or update any needed documentation to the repo
• 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)

Subgroup depicted as Icon + tooltip

Screenshot with prod analytics

[github-actions]

Preview Firebase Hosting URL: https://mobility-feeds-dev--pr-832-kjr23xpl.web.app

[emmambd]

@Alessandro100 Can we add Flex as a subgroup? It's all features in Flexible services except Continuous Stops

Also to be clear, we'll use this for internal analytics, but not the public facing search (as discussed in #806 - there's alignment in using tags for extensions like Flex and Fares v2 outside of features)

[emmambd]

@Alessandro100 It looks like the changes to features from FEATURES.md are not reflected here: https://github.com/MobilityData/gtfs-validator/blob/master/docs/FEATURES.md#feature-migration

[emmambd]

@cka-y Any clue about why the counts have changed for the ones where only the name has changed (e.g Bike Allowed, or Text-to-Speech)

[cka-y]

@emmambd I'm not sure I fully understand the question, but here's some context that might help:

• When the name of a feature changes, it’s treated as a new feature from the metrics' perspective. It’s not considered a feature update.
• The counts can also change due to dataset updates. Metrics are always calculated based on the latest dataset, so the data for a specific feed can vary from month to month as the latest dataset changes.

Does this answer your question? If not, could you clarify or provide more details?

[emmambd]

@cka-y Your second point does - I forgot we've added a fair number of feeds and so it makes sense we're seeing significantly more text-to-speech feeds. Thanks!