Tracking issue for StatusOr Experimental API

[kannanjgithub]

Allow null?
StatusOr needs to be as expressive as a method return value, although it will generally not be null. Can potentially use StatusOr<@nonnull> in cases it should be disallowed.

Should getValue() throw if there is no value?
Go with just getValue() throwing IllegalStateException for now. Can add getValueOrNull, getValueOrDefault, getValueOrThrow as we find usecases. Seems the majority case should check getStatus() first.

[JarvisCraft]

While the API is still experimental, I suggest that this type is implemented as:

// With respect to minimal Java version,
// but `sealed` may be emulated using a private type in signature,
// and records are generally translatable to value-classes.
// Also MultiReleseJar may be of help here
public sealed interface StatusOr<T> {

    // Just an example of a method, although it is usually easier to just pattern-match on types.
    T value();
    ...

    record Status<T>(io.grpc.Status status) implements StatusOr<T> {
        @Override
        public T value() {
            throw new IllegalStateException("No value present.");
        }
    }

    record Value<T>(T value) implements StatusOr<T> {}
}

which is more mentally matcing the sum-type nature of this type.