Returns instance information as outputs

[BourgerieQuentin]

The old way of running instances with slabs alowed to access to instance information so I propose to return as outputs.

As example it imply hardcoded instance on benchmark uploads (https://github.com/zama-ai/tfhe-rs/blob/bd255cd958a3754ab2503486344b30a9971387fb/.github/workflows/benchmark_boolean.yml#L82)

cc @IceTDrinker

[IceTDrinker]

sure, @soonum is the owner of this here :)

[soonum]

I'll just have to make a small changes in Slab to expose these details here.