✨ Add graphql support and add first graphql endpoint

[IamPekka058]

This pull request introduces GraphQL support to the GitHub client library and adds the ability to delete issues using the GitHub GraphQL API. The main changes include adding a new GraphQLService, updating the GitHub client to expose GraphQL functionality, extending the IssuesService with a deleteIssue method, and adding unit tests for issue deletion.

GraphQL Integration:

• Added a new GraphQLService class (lib/src/common/graphql_service.dart) to handle GraphQL queries and mutations, using the graphql package.
• Updated the GitHub client (lib/src/common/github.dart) to include a graphql getter, providing access to the new GraphQL service. [1] [2] [3]
• Added the graphql package as a dependency in pubspec.yaml.

Issue Deletion Feature:

• Implemented a new deleteIssue method in IssuesService (lib/src/services/issues_service.dart) that deletes issues via the GraphQL API, since this functionality is not available in the REST API.

Testing Enhancements:

• Added manual mocks for GitHub and GraphQLService in the unit tests, and introduced tests for the new deleteIssue method, covering both success and failure scenarios (test/unit/issues_test.dart). [1] [2] [3]