Add support for non-blocking ("async") YAML parsing

[guillermocalvo]

Jackson-core supports non-blocking parsers since version 2.9, but YAML doesn't seem to have a non-blocking parser yet.

This is preventing us from parsing YAML request bodies directly in reactive web apps (Spring WebFlux).

These are the steps to reproduce this problem:

1. Create a YAML decoder:

   public class YamlDecoder extends AbstractJackson2Decoder {
   public YamlDecoder() {
       super(new YAMLMapper(), MediaType.valueOf("application/x-yaml"));
   }
   }

2. Add it as a custom decoder:

   @EnableWebFlux
   public class AppConfiguration implements WebFluxConfigurer {
   ...
   @Override
   public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
       configurer.customCodecs().decoder(new YamlDecoder());
   }
   }

3. Add a REST controller with a mapping that consumes YAML:

   @RestController
   public class FoobarController {
   @PostMapping(value = "/foo", consumes = "application/x-yaml")
   public void foo(@RequestBody MyCustomClass body) {
       ...
   }
   }

4. Send a YAML request. Then an UnsupportedOperationException will be thrown from JsonFactory with the message:

   Non-blocking source not (yet?) supported for this format (YAML)

[cowtowncoder]

True, non-blocking handling has so far only been implemented for JSON and Smile backends. Of other backends, CBOR should be quite easy, and XML plausible with aalto-xml.

But the challenge with YAML is that this would require SnakeYAML to support it, and then format module could use it.

So: if anyone has time and interest to work on this, it'd be interesting challenge. :)