Add helper methods to construct transactions in contract clients.

[abizjak]

Purpose

Add helper methods to contract clients that only construct transactions, leaving sending to the client.

This is useful in cases where transactions have to be resubmitted in case of failure (e.g., network failure).

It came up in the work on the some-issuer service.

Checklist

• [ ] My code follows the style of this project.
• [ ] The code compiles without warnings.
• [ ] I have performed a self-review of the changes.
• [ ] I have documented my code, in particular the intent of the hard-to-understand areas.
• [ ] (If necessary) I have updated the CHANGELOG.