Now that the repository API formally supports creating repositories

backed by hosting services, it's important to communicate the fields

that must be set for each repository.
This change enhances the Hosting Service API to show the repository

plans, consisting of their ID, display name, and list of fields. This

isn't comprehensive. The list of fields doesn't include any type

information, as we'd need to be careful how we represent that from form

fields, but they do include the field name, display name, required flag,

and help text. This should be enough at this stage for API consumers.

Unit tests passed.
Manually verified the results in the payload.
Built the docs and checked that the payloads contained the information as

well.