feat: Support federation v2 directives

[Mistat]

Issue

Support federation v2 directives

TL; DR

Available Federation v2 directives:

• @external - Marks a field as owned by another service
• @extends - Allows extending types from other services
• @requires - Specifies required fields for resolution
• @provides - Specifies fields that will be provided
• @key - Defines entity keys
• @shareable - Allows fields to be resolved by multiple services
• @override - Takes responsibility for a field from another service
• @inaccessible - Marks fields as inaccessible to consumers
• @link - Links to external schemas
• @composeDirective - Preserves custom directives in supergraph

Task summary / Change details

This pull request introduces GraphQL Federation v2 support to the library, adding new directives and enhancing schema capabilities. Key changes include the implementation of Federation v2 directives, updates to directive definitions, and extensive testing to ensure compatibility and correctness.

Federation v2 Support:

• README.md: Added documentation and examples for using Federation v2 directives, including @external and @extends.

Directive Enhancements:

• directives.go: Defined AllDirectives and FederationDirectives variables to include Federation v2 directives. Added detailed descriptions for @external and @extends directives. [1] [2]

Testing Improvements:

• directives_test.go: Added tests for @external and @extends directives, verifying their names, descriptions, locations, and arguments. Also tested the inclusion of all Federation directives in FederationDirectives.
• sdl_parser_test.go: Added tests to ensure correct parsing of Federation directives and their usage in SDL, including validation of @external and @extends directives on fields and types.