Find a way to use Unstrutured *without* unsafe

[senekor]

This is an idea to keep using Unstructured in our control plane API without using unsafe and without lifetime proliferation. Basically, we keep track of which bytes have already been consumed ourselves (with the help of Unstructured::take_rest) and construct new Unstructured values over the remaining bytes for each call to the control plane API.

It involves copying all remaining bytes for each call to the API, so there is a some performance impact. This is because Unstructured may consume bytes from the start of the slice, meaning that we cannot simply truncate our vector to the length of the remaining bytes. But I was able to avoid a heap allocation per API call at least with the tmp buffer.

The API is not generic anymore, because I had issues with the generic lifetime parameter. Maybe it could've been solved, I'm not sure. But I think it wouldn't be too much effort to write one function per data type to be requested if we can still use arbitrary internally.

A consideration about the desired properties of the control plane:

It means that we could later drive the mechanism with some other source of decisions; we're not tied to arbitrary.

I think this would be covered by this approach. We can always create a new function that doesn't make use of arbitrary and uses the vector of bytes as a source of decisions directly.