Use version defines to allow settings to know if addressables installed

[Ale1]

by using asmdef version defines, the package can identify if end-user has unity's addressable package installed.
This allows for adding Addressable-only advanced features that compile only if the addressables package is present.

[starikcetin]

While I agree that providing addressable-specific functionality is a good idea, I am not planning on adding that logic to this library. I want to keep this library as simple as possible in order to make it become a global invariant component for every Unity project.

Instead, I am planning on developing a separate library that deals with providing runtime metadata information about any asset in the context of addressables, such as whether it is marked as addressable, what group it belongs to, its addressable key, etc.

Then, the two libraries used together will allow in runtime to determine, for example, whether to load/unload the scene through Addressables or SceneManager.