Rename FileOfflineStore to DaskOfflineStore

[tokoko]

FileOfflineStore uses dask under the hood and is in principle no different from any other offline store implementation that can work with file-based data formats. The name doesn't currently reflect that because the underlying execution engine was switched from pandas to dask later on. While the name is probably not important, I think the name change will be valuable to highlight that feast doesn't require one-to-one relationship between DataSources and OfflineStores. The related FileSource class (which should NOT be renamed) can be made to be used from other offline stores, for example SparkOfflineStore.

For backwards-compatibility, we can keep all user-facing classes (FileOfflineStore and maybe also FileRetrievalJob, FileOfflineStoreConfig) as aliases pointing to their new counterparts.