Datagen API improvements

[robertbastian]

#3564

• Paths in config files are now interpreted relative to the config file
• The config serde spec is now cfg-free, as that turned out to be quite confusing. The CLI now reports missing features errors
• Legacy testdata gen is now run on CI
• provider_baked::Options::use_separate_crates now defaults to true, as that's the mode required for compiled data
  • the config uses use_meta_crate, as we want to default to false
• All hello world provider testdata is now generated from configs and checked in CI