Add graphql-dotnet implementation with initial capabilities

[damienpontifex]

• First work on graphql-dotnet Federation Compatibility Testing Support #34
• Multi key support doesn't exist. Error message from graphql-dotnet when multiple @key defined on the product entity
• Would be happy to merge while investigating support for other capabilities and how they're implemented

Results from local run:

Framework	_service	@key (single)	@key (multi)	@key (composite)	@requires	@provides	@ftv1
graphql-dotnet	✔️	✔️	❌	❌	❌	❌	❌

Some insights:

• multiple @key directives https://github.com/graphql-dotnet/graphql-dotnet/blob/aa8158cfcf30b7db0eda8e1cf8e6359008891990/src/GraphQL/Utilities/Visitors/AppliedDirectivesValidationVisitor.cs#L93
• Federated tracing Federated tracing support graphql-dotnet/graphql-dotnet#2351