add getter function for contract_id

[Salka1988]

I added :

    pub const fn contract_id(&self) -> Option<&ContractId> {
        match self {
            Self::Panic { contract_id, .. } => contract_id.as_ref(),
            _ => None,
        }
    }

in order to use it in tx/receipt.rs to extend schema.sdl with the new `contract_id' field.