Allow UDT mappings to be defined before a session is created

Currently, UDT mappings are exposed via a Session property.

With CSHARP-916, users should be able to inject a session instance into their DI framework of choice without having to explictly wait for a connection to be open. However, any user that is currently defining a UDT mapping will not be able to do that because calling session.UserDefinedTypes.DefineAsync(...) will wait for a connection to be open.

We can add a way to provide the UdtMap objects in the builder and have the driver call DefineAsync() for each UdtMap during session initialization.