Enable rename of fields in proto definitions

[devdinu]

We can't change bigquery schema once created specifically renaming field, but in protobuf context renaming field is allowed.

Since the data type is not changed, we should be able to update the ColumnMapping without breaking the data flow.

message order {
  string order_number = 1
}

becomes

message order {
  string order_no = 1
}

beast should update its column mapping and push the same field in bigquery order_number