Be able to update a single dependency

[sergigp]

If I understood it correctly, update updates all dependencies in protofetch.toml and generates new revision in .lock file. It would be cool to have a way to only update a single dependency. With this we will be able to rely on master/main branches in .toml file and just update to the last version a single dependency. Right now we need to update all dependencies if we want this workflow and I think this is not something that you really want.