generateAgreement

Generate a partial transaction representing an agreement (command) to this deal, allowing a general deal/agreement flow to generate the necessary transaction for potential implementations.

TODO: Currently this is the "inception" transaction but in future an offer of some description might be an input state ref

TODO: This should more likely be a method on the Contract (on a common interface) and the changes to reference a Contract instance from a ContractState are imminent, at which point we can move this out of here.