Add `account_id::validate` check in `ownable::transfer_ownership`

[PhilippGackstatter]

The ownable::transfer_ownership logic does not check account ID validity, but probably should as a basic sanity check. During the big-endian to little-endian migration, I had suffix and prefix accidentally reversed (which is not a valid account ID), but transfer_ownership accepted the new owner regardless.

This should be as simple as adding this at the beginning of the procedure:

dup.1 dup.1 exec.::miden::protocol::account_id::validate

cc @onurinanc @afa7789 for consideration

[onurinanc]

Thanks @PhilippGackstatter for opening the issue!

[afa7789]

I will fix this.

fun fact: this type of situation is avoidable with the ownable2steps

[onurinanc]

Hey @PhilippGackstatter we consider to have Ownable2Step instead of Ownable. So, this week, we'll be opening a PR to remove the Ownable and add the Ownable2Step module. So, it would be better to close this issue for your consideration.

[PhilippGackstatter]

Agreed that Ownable2Step would fix this situation, though it may still be good to run ID validation as it catches an error earlier. Will close this issue as it should be covered by the new impl then. Thank you!