Introduce UserDefinedObject

[flobernd]

The specification uses Dictionary<string, UserDefinedValue> in many places to indicate that the server returns an "arbitrary" JSON object.

This PR introduces a new alias type UserDefinedObject that should instead be used for this purpose, because:

1. The descriptive name ensures that the semantics are clear
2. The alias type allows generators to replace the type with a more natural one, if needed (see e.g. Provide a helper for reading selected fields from search response elasticsearch-net#8758)

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[qn895]

ML UI: LGTM 🎉

[flobernd]

@l-trotta @Anaethelion Would you be fine with backporting this to 8.19 and potentially 9.2?

[l-trotta]

yep java resolves all aliases so this changes nothing for me

[Anaethelion]

Nope, we need proper handling for this first, this breaks the API badly.
I'll add an issue in the generator to handle this.