Extract all nested anonymous types into distinct named components.

[tobio]

There's a few generator related issues we're hoping to avoid with this task:

• These nested types can be defined as anonymous structs in Go (see this discussion). Whilst they're workable, it's painful and makes the code really quite unreadable.
• https://github.com/oapi-codegen/oapi-codegen/issues/1665 and a myriad of other anyOf/oneOf related issues with oapi-codegen

Ideally we can fix this in Kibana itself, but may want to do something in transformSchema in the meantime as well.