Allow caching for Optional Value

Start by tracing the Cache property-wrapper API and the validCacheExists and cache operations shown in the issue. Determine how cached nil can be distinguished from no cache while preserving a manageable migration path, then verify that Optional values support the proposed flow and that existing non-Optional caching still works.

There is a use case where the result of fetching is nil.
Now nil means no cache, so when caching an Optional value, there is no distinction between no cache and cached nil .

struct R {
    func fetch() async throws -> Data {
        @Cache(key: "my_optional_data", lifetime: .duration(3600))
        var data: Data?

        if data == nil { // no cache or cached nil?
        }
    }
}

I want to extend the following API to support Optional values.

struct R {
    func fetch() async throws -> Data {
        @Cache(key: "my_data", lifetime: .duration(3600))
        var data: Data

        if data.validCacheExists { 
            return data 
        } else {
            let newData = try? await fetchNewData()
            cache(newData, into: data)

            return newData
        }
    }
}

struct R {
    func fetch() async throws -> Data {
        @Cache(key: "my_optional_data", lifetime: .duration(3600))
        var data: Data?

        if data.validCacheExists { 
            return data 
        } else {
            let newData = try? await fetchNewData()
            cache(newData, into: data)

            return newData
        }
    }
}

This will be a breaking change, so it doesn't have to be implemented exactly as proposed.
But the migration should not be difficult.