[FR] DynamicLinks to fire event when Install Attribution request does not obtain a link

### Feature proposal

* Firebase Component: DynamicLinks

After installing an app from the app store, the Install Attribution request returns whether a dynamic link is available and eventually fires DynamicLinks.DynamicLinkReceived if a link is returned. At the moment there does not seem to be a method or event that would allow us to detect the absence of a link after the Installation Attribution request.

It would be good to know if a dynamic link was not returned by the Install Attribution request, as there are use cases for new installs e.g. invite or (specifically) no invite that could push users down different code paths depending on whether a link was received or not.

Having an event e.g. DynamicLinks.InstallLinkNotReceived or something similar would be useful to bubble up from the lower level sdks

This is similar to the functionality described in step 7 in this documentation for the iOS sdk code dynamicLinkFromCustomSchemeURL:url where it can handle if the url is nil
https://firebase.google.com/docs/dynamic-links/ios/receive#open-dynamic-links-in-your-app

Presumably this would need to be somewhere in the pods code like: FIRDLDefaultRetrievalProcessV2.FIRPostInstallAttributionCompletionHandler