extensions: Remove read_into_defaulted_vector() to let caller pass pNext

[MarijnS95]

Depends on #967

We've often discussed pNext chains in output arrays (i.e. #465, #588, #744), and I always wondered why read_into_defaulted_vector() still existed: turns out this helper function was only hiding a few more remaining cases where the caller was previously not able to manually extend the pNext chain to request arbitrary more structures to be filled with information.

Replace these remaining cases with a separate _len() getter, and have the main function take a caller-allocated &mut [T] slice where they can initialize the target struct including pNext pointer chains.

[MarijnS95]

Still need to do some code archaeology why this didn't get fixed in the past. We seem to have discussed it in #465 yet this helper made it in anyway (on a struct that has pNext...) and future PRs followed to reuse it in other extensions...

As well as a changelog entry.

[MarijnS95]

Looks like the Rust 1.83 update finally hit us. I already started looking into omitting these lifetimes but it's "hard" because the builder pattern in the generator doesn't trivially know if it's going to be referencing one from the impl block, if i.e. we have a pNext chain without structextends referencing it for example. Let's see what can be done.