Expose provider::Baked as per-formatter Fork providers

[Manishearth]

The current scenario with baked vs other providers is that you have roughly equivalent functionality, except there is no easy way to do fancy data provider stuff with baked data. You can't easily mix baked data with buffer data, or "intercept" the baked data load to understand resolved locales as needed by some strategies for storing resolved options (#5900).

A concrete use case would be wishing to use a hotloadable buffer provider for locale data, and baked data for closed sets of data (like digits data, UCD data, etc).

We do have provider::Baked in each crate but it's not all the data needed by the type. It would be nice if each formatter type had a const fn get_baked_provider() -> impl DataProvider ... that produces a baked data provider that works for that type. Typically it will be provider::Baked, or it will be something like ForkByKeyProvider<provider::Baked, othercrate::provider::Baked>

cc @sffc @robertbastian

[Manishearth]

@sffc noted that the ForkByKey provider doesn't implement DataProvider the way we want it to because of specialization, but we could instead use a macro to generate a custom type with a host of DataProvider impls

[Manishearth]

We can also do this in the metacrate, using the registry. Per-formatter baked providers are not strictly necessary.