lockup: Transferable vesting accounts

[armaniferrante]

There is some demand for the ability to transfer the beneficiary of a vesting account to a new account. https://discord.com/channels/739225212658122886/739236663926063145/823206958206812166.

One challenge here is that lockups have an optional "realizer" that determines if the lockup withdrawal can be executed. So when transferring locked token accounts, the realizer condition needs to be respected. For stake rewards, for example, the realizer is the condition that the user has zero stake pool tokens. So when transferring locked tokens, even if the user's wallet is the same, the stake amount might be different, which would be undesireable if it means the user can unstake less tokens than the reward for which is supposed to be dependent on the stake tokens. One potential solution would be to just allow the transfer if the user has at least as much stake tokens in the new stake account as the user has in the old stake account.