Leases of assets such as ships and colonies

**What do you want to see?**
The ability to trade something temporarily; it would return to its original owner after a specified period of time

**How would it help players?**
Players could set up things like mercenary fleets that they lease out to other players, or they could borrow ships or colonies to refit them with their own technology for another player.

**Alternatives**
This can already be done via the trade system, but there's currently no way to force the assets to be automatically returned. An explicit end date on the trade would fix that.

**Screenshots and mockups**
N/A

**Things to consider**
What happens if the assets are destroyed or damaged during the lease? Should there be some form of collateral or should it be lender beware?

**Related issues**
N/A