defined types and constants in different packages

[hilariocoelho]

Moved some notifications related types and constants to different packages in order to make it easier for 3rd party services to implement gorush.

Since gorush package has some init functions (here and here), this package should not be directly imported by other projects. There is even an init function that registers prometheus metrics and may crash the project that tries to import the package.

This PR addresses this situation by moving types and constants values to different packages that only imports safe packages (appleboy/go-fcm) which don't have any init functions.

Having structs and constants defined in importable packages makes life easier for projects that need to call GoRush API by having already created types and constants values that are fully supported by GoRush itself.