store: Add separate store-object types for the store writer API.

store: Add separate store-object types for the store writer API.

The argument list of some of the callbacks has already gotten kinda long and
we'll want to add more (backends, etc.). Introduce new types which provide all
meta-data of the respective store types.

Also, only use a single callback to store all kinds of attributes, similar to
how it was already done in the "proto" module.