Add a custom ID for all Objects on Xero.

We're building a system to manage and sync data across multiple platforms, and we're having many issues keep data in sync with bulk inserts and writing back to our own IDs. For example when bulk inserting payments we cannot match them to the payments on the system as there is no unique number we can pass through and no other field is reliable. Due to the API rate limits we're forced to use Bulk inserts so we cannot update on an individual basis.