Contracts: return `proof_size` with `ContractResult` from dry-runs

[ascjones]

Currently, in order to choose a value for gas_limit a UI would first dry-run the extrinsic via state_call and use the resulting ContractResult::gas_required as a parameter to the actual extrinsic.

Now that weights are two dimensional and also include proof_size, we should add that to the ContractResult in order for UIs to correctly populate that value, without having to manually supply a value.

Rel paritytech/subxt#679

[burdges]

As an aside, actual runtimes could change since other transactions could impact Merkle proof CPU time, but not sure this impacts weight. It won't change much usually though..

[athei]

Yes state can always change in-between and make the gas estimation stale. In practice it works well.