feat(sui): add gRPC migration level 8 for transaction execution

Migrate execute_transaction and estimate_gas_budget to gRPC behind
GRPC_MIGRATION_LEVEL_TRANSACTION_WRITES (level 8). Adds
DualClient::execute_transaction_grpc using the ExecuteTransaction RPC
and DualClient::simulate_transaction_for_gas_grpc using
SimulateTransaction for gas estimation. Includes conversion helpers for
TransactionEffects (BCS), ChangedObject → ObjectChange, and proto
Owner → sui_types Owner.

Author: wbbradley