Feat: OpenAPI spec for Provider

[thekaveman]

View the Provider API docs: https://openmobilityfnd.stoplight.io/docs/mds-openapi/

• Defines all the common field types used in MDS (e.g. uuid, timestamp, vehicle_type)
• Defines all the common data models used in MDS (e.g. vehicle, event, trip)
• Defines mode-specific mixins for each data model
• Fully describes Provider API endpoints, request, and response schemas
• Defines valid state machine transitions for event per-mode, with examples RE https://github.com/openmobilityfoundation/mobility-data-specification/issues/792
• Updates data payload format RE https://github.com/openmobilityfoundation/mobility-data-specification/issues/841
• Aligns event_types RE https://github.com/openmobilityfoundation/mobility-data-specification/issues/845
• Adjusts accessibility_attributes requirement RE https://github.com/openmobilityfoundation/mobility-data-specification/issues/847
• Normalize fare_attributes.payment_types RE https://github.com/openmobilityfoundation/mobility-data-specification/issues/844
• Adds OpenAPI schema linting with Spectral for local development and in GitHub

Once this is merged into main, I'll make that the default branch in the Stoplight project.

Closes #6 Closes openmobilityfoundation/mobility-data-specification#792

[thekaveman]

Merging this now so I can have a clean base for the agency work and beyond. Any additional fixes that come up based on spec review can be added as follow-on PRs.