feat: topic client identity auth

[richard1927]

Adding the option to add topic clients that use identity authentication

[Copilot]

Pull Request Overview

This PR adds support for identity-based authentication when creating Azure Service Bus topic clients. Previously, only connection string authentication was supported.

• Introduces a new overload of AddTopicClients that accepts TokenCredential for identity authentication
• Refactors common logic into a private helper method to avoid code duplication
• Improves topic name validation by filtering out null/whitespace entries and removing duplicates

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