Openapi spec to typescript client/sdk

[losolio]

Our openapi-typescript-codegen is now deprecated and we should move on. The author of the old package points at @hey-api/openapi-ts as the succesor.

Did try to make an experimental client for the registrations-api in the repo.

@meinte could you take a look into this, and also feel free to search for some alternatives.

Note that to make place for possible other sdks/clients for the new content api for payload - I am thinking on renaming the sdk to something around registrations.

[losolio]

Might be relevant: https://openapi-ts.pages.dev/introduction