Accept multiple JSON serialization contexts in DI

[mtmk]

Proposed change

public NatsBuilder AddJsonSerialization(params JsonSerializerContext[] contexts)
    => ConfigureOptions(opts =>
    {
        var jsonRegistry = new NatsJsonContextSerializerRegistry(contexts);
        return opts with { SerializerRegistry = jsonRegistry };
    });

...or collect them to add them in the end during build:

public NatsBuilder AddJsonSerialization(params JsonSerializerContext[] contexts)
    => _contexts.AddRange(contexts);

cc @rickdotnet