Introduce IDPoPProofTokenFactory

[bhazen]

What issue does this PR address?
Added IDPoPProofTokenFactory to allow for customizations to how proof tokens are generated. This came from a community request for more flexibility.

As part of these changes, DPoPProofTokenFactory was renamed to DefaultDPoPProofTokenFactory to make it clear it is the default behavior.

New extensions methods were added for when a user wishes to supply their own implementation of IDPoPProofTokenFactory. Otherwise the existing extension methods will continue to work for existing code

[Copilot]

Pull Request Overview

This PR introduces a new interface IDPoPProofTokenFactory to provide flexibility for customizing DPoP proof token generation, addressing a community request. The existing DPoPProofTokenFactory is renamed to DefaultDPoPProofTokenFactory to clarify its role as the default implementation.

• Introduces IDPoPProofTokenFactory interface for proof token customization
• Renames DPoPProofTokenFactory to DefaultDPoPProofTokenFactory
• Adds new extension methods accepting IDPoPProofTokenFactory while maintaining backward compatibility

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
IDPoPProofTokenFactory.cs	New interface defining contract for proof token factories
DefaultDPoPProofTokenFactory.cs	Renamed class implementing the new interface with default behavior
ProofTokenMessageHandler.cs	Updated constructor to accept interface instead of concrete type
OidcClientExtensions.cs	Added overloads accepting interface while preserving existing string-based methods
DPoPTests.cs	Updated test to use new factory pattern

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