Since we will need to add other api endpoints, e.g. `contentSourcesApi` for
the on-prem frontend, this PR restructures the store directory to make future
changes more manageable.
Set the option to generate union undefined types for objects that have
optional items. This will allow us to enable `exactOptionalPropertyTypes`
in our tsconfig [1].
[1] This is a recommended setting, see:
https://www.typescriptlang.org/tsconfig/#exactOptionalPropertyTypes
