I found `JsonContentPolymorphicSerializer`, but th...

# serialization

[diesieben07]

I found JsonContentPolymorphicSerializer, but that doesn't work if I also need to support Yaml (or similar) input. And it is a hard fail without a way to provide a separate implementation for yaml. It would be nice if we could have something like a PeekableDecoder with a way to try (and then rewind) a serialization.