Fork solana-account for copy-on-write account storage

[bmuddha]

Goal

Fork solana-account and add the account representation changes needed by accountsdb.

Why

accountsdb depends on a forked solana-account that can represent accounts in a copy-on-write layout and support the storage/routing model used by the engine.

Scope

• fork solana-account
• add the account COW representation and related helpers needed by accountsdb
• keep the API and layout aligned with the engine’s storage model