fix: ability to configure interface entities

[dariuszkuc]

Adds new KeyInterfaceAttribute that allows specifying @key directive on GraphQL interfaces.

Resolves: #5

[dariuszkuc]

Unsure why but it didn't work when I tried to update KeyAttribute to a generic Attribute and manually applying the directives in the type interceptor. Instead of spending too much time digging into this, this seemed like a simpler fix to unblock the feature.

[github-actions]

Apollo Federation Subgraph Compatibility Results

Federation 1 Support	Federation 2 Support
_service 🟢 @key (single) 🟢 @key (multi) 🟢 @key (composite) 🔲 repeatable @key 🟢 @requires 🟢 @provides 🟢 federated tracing 🔲	@link 🟢 @shareable 🟢 @tag 🟢 @override 🟢 @inaccessible 🟢 @composeDirective 🟢 @interfaceObject 🟢

Learn more:

• Apollo Federation Subgraph Specification
• Compatibility Tests