Suggestion: Remove optional dependencies from the NuGet specification

[ernestoyaquello]

In the version of Localytics for native Android, some optional dependencies are not added by default. Instead, the user is expected to add them manually if they want Localytics to make use of them. For example, com.google.firebase:firebase-messaging is not added as a dependency automatically, and it is the user's responsibility to add it if they decide to set ll_fcm_push_services_enabled to true.

It would be great if this same idea were applied in the Xamarin version of the library. Right now, all the optional dependencies are added by default with the NuGet package even if they are not needed, which is not ideal.

[tomcurran]

I've implemented a possible approach to this by removing Firebase and Google Play Services dependencies from library project that aren't mandatory. They are optional like the native library. PR #62 needs to go through first before creating a PR for it, example here tomcurran#2