Add support for schema generation with System.Text.Json.Schema

[ldeluigi]

Use Case

System.Text.Json.Schema exposes the GetJsonSchemaAsNode method, acting on a JsonSerializerOptions configured with ConfigureForNodaTime

Expected Behavior

The Json schema is generated and valid

Actual Behavior

NodaTime properties schemas are empty. They miss the "type" property which is mandatory.

[jskeet]

@ldeluigi: I think it very unlikely that I'll ever implement this. If you would like to do so, I'll be happy to review a pull request.

[ldeluigi]

Given the lack of support from System.Text.Json for custom types, we opted to use only primitive types or builtin time data structures and then convert everything to nodatime after the deserialization