schema: Annotations to oneOf and add oneOfEnumSelectorField

[ghost]

openownership/lib-cove-bods#64

Overview

What does this pull request do?

Changes part of schema to be technically correct and add a hint field so we get better validation messages

How can a reviewer test or examine your changes?

No changes to data model. To see changes in validation messages:

• run Cove locally
• make sure you use version 0.27 of https://github.com/OpenDataServices/lib-cove
• follow instructions in https://github.com/openownership/lib-cove-bods#updating-schema-files-in-data to make your cove use the new compiled schema files
• try the broken sample data at the top of Unhelpful validation error message lib-cove-bods#64

Who is best placed to review it?

Already discussed with @rhiaro and @kd-ods

Relates to issue: openownership/lib-cove-bods#64

Translations

• ⛔ I've notified the translation coordinator of any new strings that will need
  translating. See: https://openownership.github.io/bods-dev-handbook/translations.html - There are none.

Documentation & Release

• 🆗 I've thought about how and when this needs to be released. See:
  https://openownership.github.io/bods-dev-handbook/standard_releases.html I think this is possible to release in 0.2.1 and that would be better than 0.3 just because users will see the better validation messages faster
• 🆗 I've updated the changelog, if necessary - done
• ⛔ I've added or edited any related documentation. See: https://github.com/openownership/bods-dev-handbook/blob/master/style_guide.md - there is none

[rhiaro]

Minor changelog formating pedantry changes, otherwise fine.

Note: I opened #379 to track eventually replacing this with a different way of passing the information to the validator.