data provider: remove prefer_database_credentials

Google Cloud Storage credentials are now always stored within the data
provider.

Added a migration to read credentials from disk and store them inside the
data provider.

After v2.3 we can also remove credentials_path

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

vfs/azblobfs.go

@@ -70,7 +70,7 @@ func NewAzBlobFs(connectionID, localTempDir, mountPath string, config AzBlobFsCo
 		ctxTimeout:     30 * time.Second,
 		ctxLongTimeout: 90 * time.Second,
 	}
-	if err := fs.config.Validate(); err != nil {
+	if err := fs.config.validate(); err != nil {
 		return fs, err
 	}