viewmodel-savedstate: Add delegate for SavedStateHandle

[osipxd]

Draft:

/** Creates delegate to read value from [SavedStateHandle]. */
inline fun <reified T> SavedStateHandle.value(key: String? = null): ReadOnlyProperty<Any?, T> {
    return ReadOnlyProperty { _, property ->
        if (null is T) {
            get<T>(key ?: property.name) as T
        } else {
            checkNotNull(get(key ?: property.name))
        }
    }
}