[savme]

Adds an execTerminal runtime class feature for classes whose shell sessions can have an interactive terminal. Unikernel instances run exec on pipes with no PTY, so -it sessions and the portal shell get no prompt, echo or signals; this lets the unikernel class serve exec without claiming support for terminals.

Reported to the vendor. The unikernel class gets execTerminal once the runtime supports terminals.

This change is intended to unblock one-off exec on unikernels.

• New execTerminal feature value, described as "interactive terminals in shell sessions"
• CEL rule: a class declaring execTerminal must also declare exec
• Nothing enforces it yet. Admission and the portal start requiring it in a follow-up, once general-purpose declares it

Part of datum-cloud/unikraft-provider#247