Add `convertValue()` method to `ObjectReader`

`ObjectMapper` has a convenient `convertValue` method which is not currently propagated to `ObjectReader`. Adding this method would be useful in situations where one needs to add custom attributes for a deserialization operation -which is only possible with `ObjectReader` since there is no readWithAttributes option for the `ObjectMapper`. Thus,currently one must first convert the the source object to a `JsonNode`, then pass that node to the `Object Reader`. If I understand correctly, the extra step would be unnecessarily slow because the fast `TokenBuffer` conversion method cannot be used.