[API review] Preserve OpenAPIAny and mark as obsolete to make migration to 2.0 easier.

[RachitMalik12]

Requestor: @baywet

To make the migration for the OpenAPIAny type to JsonNode easier, preserve the types, mark as obsolete and add the implicit conversations to JsonNode.
@RachitMalik12 to research if a progressive migration path is needed based on community feedback or if we should proceed with the "bandaid" approach.

[darrelmiller]

Also add implicit conversion from JsonNode to OpenAPIAny so that users can create an extension using a JsonNode without having to be aware of OpenApiAny.

[RachitMalik12]

Related: #2151

[darrelmiller]

The implicit operator doesn't work because the dictionary is expecting an IOpenApiExtension, not an OpenApiAny. So, the compromise is give OpenApiAny a better name, JsonNodeExtension

            document.Extensions.Add("x-foo", new JsonNodeExtension(new JsonObject()
            {
                ["bar"] = "hey",
                ["baz"] = "hi!"
            }));

This pattern matches nicely how custom extensions work, and only requires adding one more object initialization.

As for maintaining all the OpenApiAny, OpenApiString, OpenApiInteger classes as obsolete, we are going to defer that work and only add it based on feedback from customers. No-one has raised this as a concern during preview and this kind of mechanical translation is probably something that LLMs are very good at doing.