Allow custom ISerializer implementations to be used with SDK

[brendonofficial]

Currently, the SDK has its own internal instance of NewtonsoftJsonSerializer with hardcoded serializer options configured. This makes it impossible to manipulate the behaviour of (de-)serialized xp objects.

There are a few possible options to address this:

• use ISerializer instance from DI container (but this possibly interferes with use of OrderCloudContractResolver)
• use internal serializer for OrderCloud objects, but allow custom ISerializer implementation to be used with xp objects, defaulting to the internal if one is not configured