add index to GSFA response

[0xMavi]

Summary

• Adds a transaction_index field to getTransaction and getSignaturesForAddress RPC responses
• The index represents the transaction's 0-based position within the block's flattened transaction list
• Enables clients to determine transaction ordering within a block without fetching the entire block

Changes

• Added index: u32 field to ConfirmedTransactionWithStatusMeta and ConfirmedTransactionStatusWithSignature structs
• Modified find_transaction_in_slot in blockstore to return (VersionedTransaction, u32) tuple
• Updated find_address_signatures_for_slot to return index alongside slot and signature
• Added transaction_index to RpcConfirmedTransactionStatusWithSignature and EncodedConfirmedTransactionWithStatusMeta response types
• Updated BigTable storage layer to propagate index through transaction lookups
• Updated CLI code to ignore new struct field

Testing

• Updated existing unit tests in blockstore.rs to verify index values are correctly returned
• Tests confirm index matches expected transaction position within entries