Added appropriate super-traits to `Enum`.

The `Enum` trait now requires several other traits, including `Singular`, `Into<i32>`, `Copy`, `Proxied`, `EntityType`, and `SealedInternal`. This simplifies bounds in functions that operate on types implementing `Enum`, as they no longer need to explicitly list `+ Singular` and other common requirements.

PiperOrigin-RevId: 888286136