[REQ][CSHARP] Make TokenProvider abstract method GetAsync protected for override

[alec-petersen]

Fixes #22601

This PR:

• Makes GetAsync in TokenProvider protected internal so that users can override it when they create their own TokenProvider.
• Removes the ArgumentException to allow for TokenProviders that generate tokens at runtime instead of having them ready at the beginning of execution.

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[cubic-dev-ai]

No issues found across 2 files

[alec-petersen]

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