Custom implementation of convertValue for input types

[paulbakker]

We use Jackson's convertValue to convert the Map of input values created by graphql-java to a Java class for ease-of-use in data fetchers.
Using Jackson's convertValue has always been problematic. It serializes the input Map and then deserializes it again into the target Class. In this process, it doesn't know that graphql-java already took care of deserializing scalar values.

This means the following happened:
Query input (as String) -> deserialized (using scalars) to Map by graphql-java -> Jackson serializes the value again, without using scalars, and deserializes to a Class, again without using scalars.

In most cases, this happened to work, because Jackson could "accidentally" successfully serialize/deserialize to the same value. In some cases, such as more complex scalars (e.g. data/time), this didn't result in the correct value.
Even though it worked "most of the time", it has always been fundamentally incorrect.

Changes in this PR

In this PR a custom "converter" is added. Note that the converter only maps a Map to a Java type, it doesn't do any deserialization. The deserialization is done by graphql-java.
It accounts for both Java and Kotlin classes because they are instantiated differently through reflection.
We already had good test coverage for @InputArgument and all tests are passing, making this mostly a change that users won't notice, except in the special cases that previously didn't work.

Fixes: #70

[berngp]

Changes look good to me. I'll do another pass but I think we should merge and have an RC.