JsonSchemaExporter: Consider setting readOnly/writeOnly

From the spec:

https://www.ietf.org/archive/id/draft-bhutton-json-schema-validation-01.html#section-9.4

I think these should map to the internal CanSerialize/CanDeserialize properties on JsonPropertyInfo?

The logic should be along the lines of:

```csharp
if (jsonPropertyInfo.CanSerialize && !jsonPropertyInfo.CanDeserialize)
{
jsonSchema.WriteOnly = true;
}
else if (jsonPropertyInfo.CanDeserialize && !jsonProperty.CanSerialize)
{
jsonSchema.ReadOnly = true;
}
```

@eiriktsarpalis Does that make sense?

Search for JsonSchemaExporter and JsonPropertyInfo first, then inspect the existing property-schema export path and related tests. Add coverage showing how CanSerialize and CanDeserialize affect readOnly and writeOnly, and consider the work complete when the expected schema flags are verified for both directions.

Written by the indexing model from the issue text.