fix: refactor RSA bits to remove unneeded classes

[kevmoo]

And flagged members to ensure they are not exposed publicly

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PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
googleapis_auth	None	2.2.0	2.2.1-wip	2.2.1-wip	✔️

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Package	Status
googleapis_auth	✔️ All dependencies utilized correctly.

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[kevmoo]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors parts of the RSA implementation to improve code structure and encapsulation. The RSAAlgorithm class, which was a container for static methods, has been removed, and its methods are now top-level functions within lib/src/crypto/rsa.dart. This change aligns with Dart best practices. Additionally, the visibility of these functions and the RS256Signer class has been restricted using @internal and @visibleForTesting annotations, effectively hiding implementation details from the public API. The changes are consistently applied across the codebase, including updates to tests. Overall, this is a solid refactoring that enhances maintainability.