Porposal: Split all packages under go-storage/pkg

I propose to split all packages under go-storage/pkg into other repos.

Following packages are affected:

After this change, go-storage only maintains core interface and definitions.

Related issue: Move pkg/credential to go-credential? · Issue #617 · beyondstorage/go-storage · GitHub

Are they too simple to be a separate module?

It does. I propose to split them for introducing changes without upgrading go-storage.