Add GithubSyncService and IGithubSyncService for syncing documents with system

[afshinalizadeh]

Problem Statement

The system currently lacks integration to sync documents stored in GitHub with our internal model (CrystaDocument).

Proposed Feature

• Add a GithubSyncService and its interface IGithubSyncService.
• This service must synchronize documents in GitHub with our system using the CrystaDocument model.
• For fetching documents from GitHub, leverage DocumentRepositoryInMemory.GetDocumentsAsync (returns all documents).
• The service should retrieve documents from GitHub and perform the necessary synchronization logic so our system stays up to date.

Relevant Classes

• DocumentRepositoryInMemory with the GetDocumentsAsync method is available for fetching all GitHub documents.

Acceptance Criteria

• GithubSyncService and IGithubSyncService added to the system.
• Service fetches documents from GitHub using GetDocumentsAsync.
• Documents are synchronized with the system using the CrystaDocument model.
• Well-documented code and clear integration points.

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If additional details or architectural constraints exist, please specify. Otherwise, implementation can proceed per above.