Bugfix: Allow JSON Documents with DateTime scalars to be deserialized to Objects

[richardcresswell]

Pull Request type

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Changes in this PR

Allow the DgsQueryExecutor to deserialize JSON responses to Objects for documents containing scalar DateTime types.
Resolves #269

[paulbakker]

We have the same in GraphQLResponse in the graphql-dgs-client module, so it makes sense to support the same here.

[srinivasankavitha]

Agree with @paulbakker. We actually also do the same thing in DgsSchemaProvider.kt as well. I wonder if we can use a common object mapper, at least within the same core module.

Edit: After some discussion, doesn't seem useful to share the object mapper since they are for different use cases.