Implement `is_known` method on the `Enum` trait

This is equivalent to C++'s `_IsValid` functions. Since proto3 enums are open,
`is_known` seems to be a better name than the misleading `is_valid`.

C++-specific Protocol Buffers documentation already uses "known fields" and
"unknown enum values" expressions.

PiperOrigin-RevId: 630344180