API wraps a few high level operations of a small part of the FOLIO API, e.g.

FincConfigMetadataCollection is a single configuration entry.

LoginResponse for bl-users/login, used to obtain auth tokens.

MetadataCollectionsOpts collections options for the metadata collections API.

MetadataCollectionsResponse collects zero, one or more collection entries obtained from the API.

Doer is implemented by HTTP clients, typically.