Adopt IAuthorizationHeaderProvider2 from Abstractions 12.3.0

[4gust]

• Bump Microsoft.Identity.Abstractions to 12.3.0
• Implement IAuthorizationHeaderProvider2 on DefaultAuthorizationHeaderProvider and the public BaseAuthorizationHeaderProvider; consolidate all header-creation paths through a single BuildHeaderInformationAsync engine
• Propagate TokenAcquisitionMetadata and AdditionalResponseParameters from MSAL onto AuthorizationHeaderInformation via the existing AcquireTokenResultFactory
• DownstreamApi and MicrosoftIdentityMessageHandler prefer IAuthorizationHeaderProvider2 for mTLS PoP, with IBoundAuthorizationHeaderProvider kept as a back-compat fallback
• Add PublicAPI.Unshipped entries for the new virtuals on BaseAuthorizationHeaderProvider
• Add DefaultAuthorizationHeaderProviderV2Tests covering bearer/app/user flows, metadata + additional-response-parameter propagation, and token-binding guard

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[gladjohn]

Two minor maintainability nits on the new mTLS-PoP token-binding dispatch (not blocking).