Allow SerializationContextInitializer dependencies

In order to avoid split-packages across multiple maven modules and the resulting jars, it's necessary for Protostream to provide a mechanism for SerializationContextInitializer implementations in one module to depend on those defined in dependent modules.

For example, the org.infinispan.commons.io.ByteBufferImpl class is defined in the Infinispan commons module, but is required by several annotated Pojos in the core module. In order for the core module to compile, it is necessary for ByteBufferImpl.class to be added to the classes field in the core modules AutoProtoSchemaBuilder annotation with autoImportClasses == false or for autoImportClasses == true. Either way, this results in org.infinispan.commons.io.ByteBufferImp$_Marshaller.class being added to the core jar.