Attributions for dataset / agency / route / trip in the GTFS GraphQL API

**Goal / high level use-case**
There is a legal requirement for some of our data providers to show attributions.

**Describe the solution you'd like**
I am putting these attributions in the GTFS file. I need OpenTripPlanner to import these attributions and make them available to our clients.

**Describe alternatives you've considered**
N/A

**Additional context**
