[tsl:concurrency] Add special handling of Map functors returning StatusOr<U> results
[tsl:concurrency] Add special handling of Map functors returning StatusOr<U> results

AsyncValueRef<absl::StatusOr<T>> is discouraged and should be replaced with AsyncValueRef<T> because async value is already an container with a absl::Status or a value and Status of Status makes APIs ambiguous.

Add specializations to AsyncValueRef::Map to handle functors returning StatusOr objects.
