Add plumbing to calculate backend specific file hash for upload

This enables the backends to request the calculation of a
backend-specific hash. For the currently supported backends this will
always be MD5. The hash calculation happens as early as possible, for
pack files this is during assembly of the pack file. That way the hash
would even capture corruptions of the temporary pack file on disk.

internal/backend/b2/b2.go

@@ -2,6 +2,7 @@ package b2
 
 import (
 	"context"
+	"hash"
 	"io"
 	"net/http"
 	"path"
@@ -137,6 +138,11 @@ func (be *b2Backend) Location() string {
 	return be.cfg.Bucket
 }
 
+// Hasher may return a hash function for calculating a content hash for the backend
+func (be *b2Backend) Hasher() hash.Hash {
+	return nil
+}
+
 // IsNotExist returns true if the error is caused by a non-existing file.
 func (be *b2Backend) IsNotExist(err error) bool {
 	return b2.IsNotExist(errors.Cause(err))