Fix logger and service providers fields initialization

[guyp-descope]

The logger and serviceProviders fields of samlidp.Server object are not exported and therefor cannot be initialized when not using the default func New(opts Options) (*Server, error) ,
The fix include two things:

1. Added InitializeLogger(..) function so logger can be initialized
2. Added check for nil map before adding new serviceProvider which will initialized the map if needed.

[guyp-descope]

Hi @crewjam, any feedback on this will be much appreciated. thanks