Make AuthInterop and AppCheckInterop properties optional in Storage

[andrewheard]

The AuthInterop and AppCheckInterop instances returned by ComponentType<ProtocolName>.instance(for: ProtocolName.self, in: app.container) are nullable (e.g., if AppCheck or Auth aren't added to an app). This hasn't broken the Storage SDK because the instances are only used in StorageTokenAuthorizer, where they're nullable parameters.

#no-changelog

[andrewheard]

Note: We could consider making the method FIRComponentType
+ (T)instanceForProtocol:inContainer: return a nullable T instead.

This would force us to ensure we handle nil in cases like AuthInterop or AppCheckInterop but would require us to force-unwrap in cases like StorageProvider where we know the value won't be nil (or something is very wrong if it is).

[paulb777]

@andrewheard Yes, I think it should return a nullable T.

Instead of force unwrapping, we should do if let ... else fatalError()