Add support for specifiedBy directive

[kay-schecker]

Hi, I just added support for the @specifiedBy directive, because it is still missing and I need the support, because otherwise the validation of https://github.com/spawnia/sailor fails and blocks me.

Can you please take a review?

Todos left I was not able to fix SchemaExtenderTest.testExtendsScalarsByAddingSpecifiedByDirective during my timeframe :/

Can you support here please?

Thanks a lot, Kay