Add model name to metadata.json for every asset

[reyammer]

Would be good to add standard_v2_0 to the metadata file (example: https://github.com/google/magika/blob/main/assets/models/standard_v2_0/metadata.json) so that it's clear which model one is using without the need to rely on the folder name.

We should fix the model exporter code to add this information.