Add function to add template model-metadata-schema.json file to hub-config directory

[annakrystalli]

We do not include model-metadata-schema.json file, used to validate model metadata files, in the hubverse schemas repo because they are files we want to allow hub administrators to use flexibly to specify the metadata required for models in their hub. We have however developed a template to get them going.

At the moment, the only place the template model-metadata-schema.json exists is the hubTemplate repository. https://github.com/hubverse-org/hubTemplate/blob/main/hub-config/model-metadata-schema.json

It could be useful to have a function that hub administrators could use to add said template to their hub's hub-config directory

The function could build on usethis style templating functions e.g. usethis::use_readme_md()