add @composeDirective to sangria implementation

[oskargotte]

Updates the sangria-federated implementation with a custom directive using @composeDirective to make the compatibility test for this directive to pass.

[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 🟢