Add []byte to repo.LoadAndDecrypt and utils.LoadAll

This commit changes the signatures for repository.LoadAndDecrypt and
utils.LoadAll to allow passing in a []byte as the buffer to use. This
buffer is enlarged as needed, and returned back to the caller for
further use.

In later commits, this allows reducing allocations by reusing a buffer
for multiple calls, e.g. in a worker function.

internal/repository/key.go

@@ -184,7 +184,7 @@ func SearchKey(ctx context.Context, s *Repository, password string, maxKeys int,
 // LoadKey loads a key from the backend.
 func LoadKey(ctx context.Context, s *Repository, name string) (k *Key, err error) {
 	h := restic.Handle{Type: restic.KeyFile, Name: name}
-	data, err := backend.LoadAll(ctx, s.be, h)
+	data, err := backend.LoadAll(ctx, nil, s.be, h)
 	if err != nil {
 		return nil, err
 	}