feat: support appointments with an operation

[jwilkinson87]

This pull request introduces support for including and hydrating Appointment resources within Operation entities in the codebase. It adds the necessary data model, updates the router and response handling to support the new relationship, and provides comprehensive test coverage to ensure correct deserialization and integration.

The most important changes are:

Appointment Model and Relationship Integration:

• Added a new Appointment data class in com.ctrlhub.core.projects.appointments.response, representing appointment details and supporting JSON:API serialization/deserialization.
• Updated the Operation model to include an appointment relationship, allowing operations to reference an associated appointment if present. [1] [2]

Router and API Enhancements:

• Modified OperationsRouter to support fetching and hydrating Appointment resources as included relationships, and registered Appointment as a recognized resource type for deserialization. [1] [2]
• Extended the OperationIncludes enum to support including appointments in API requests.

Testing and CI Improvements:

• Added a new test case in OperationsRouterTest to verify that operations with included appointments are correctly retrieved and hydrated, using a new example JSON fixture. [1] [2]
• Introduced a new GitHub Actions CI workflow that runs tests on pushes and pull requests, and sends notifications to Slack for test start, pass, and failure events.

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[andrew-waters]

Good that we've got properties as another include here - that'll be handy :)