Add Support for GraphQLError-Extension data

[MaaxGr]

The GraphQL spec states:

GraphQL services may provide an additional entry to errors with key extensions. This entry, if set, must have a map as its value. This entry is reserved for implementors to add additional information to errors however they see fit, and there are no additional restrictions on its contents.
see https://spec.graphql.org/June2018/#example-90475

This should be implmeneted into this project as well.

[sangeetds]

Hi @MaaxGr I was facing a similar issue and was going through the issues section to find anything similar. I have forked this repo and trying to maintain it. I thought it would be better if you could open the same PR at https://github.com/sangeetds/KGraphQL instead of me pushing your commit from here. Thanks!

[MaaxGr]

Yeah. I will do this later. :)

[MaaxGr]

I closed my merge request as the solution of @ima9dan in #171 is better.