Need an annotation to denote proper field ordering

[creativedrewy]

As I've come to learn with Borsh serialization, the processing order of the fields is of utmost importance for proper serialization/deserialization. Unfortunately Java doesn't enforce any sort of internal ordering for its properties, so when borshj goes to deserialize, the properties might be handled in any order, thus making deserialization not work.

As a result, an annotation with an order property is likely needed for POJO fields to denote the order that they should be handled. Then when processing the fields in the readPOJO() methods, you can just sort by the annotation's order number and things will work properly. I've submitted a PR with just such an annotation for a kotlin-based implementation of borsh. You can see it here:

metaplex-foundation/SolanaKT#31

[artob]

This would be a good feature to have. Pull requests welcome!