Add `asFlow()` extension for `SharedFlow`

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Currently, kotlinx.coroutines provides:

fun <T> MutableSharedFlow<T>.asSharedFlow(): SharedFlow<T>

and

fun <T> MutableStateFlow<T>.asStateFlow(): StateFlow<T>

to hide mutability.

However, there's no convenient way to hide the "hot flow" nature and return a simple Flow<T> that cannot be cast back to the original type. I suggest to add:

fun <T> SharedFlow<T>.asFlow(): Flow<T>

This would provide a clean, zero-overhead way to expose hot flows as a plain Flow interface.

Use cases

• API design where implementation details (SharedFlow/StateFlow) should not leak.
• Preventing clients from accessing replayCache, subscriptionCount, or value.
• Consistent return types in interfaces that may use different Flow implementations.

private val _mutableSharedFlow = MutableSharedFlow<Value>()
override val values: Flow<Value> = _mutableSharedFlow.asFlow()