protected void register(final Class<? extends Io> ioClass, final Class clazz, final Object serializer) {
        if (!registeredSerializers.containsKey(ioClass))
            registeredSerializers.put(ioClass, new ArrayList<>());
        registeredSerializers.get(ioClass).add(Pair.with(clazz, serializer));
    }
Add a "serializer" for the {@code Mapper}.  Note that what is accepted as a "serializer" is implementation
specific.  An {@link Io} implementation will consult this registry for "serializer" classes
it expects so refer to the {@link Io} implementation to understand what is expected for these values.

@param clazz usually this is the class that is to be serialized - may be {@code null}
@param serializer a serializer implementation
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/AbstractIoRegistry.java