V3 TIFF : Allow additional and undefined extra samples

[JimBobSquarePants]

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Description

Backport of #2941 for V3

[Copilot]

Pull Request Overview

This PR backports support for additional and undefined extra samples in TIFF images. The changes include:

• Adding a new test image constant and theory test for extra samples.
• Adding a new enum member to handle CorelDRAW-specific unassociated alpha data.
• Updating the TIFF decoder options parser to handle the new extra sample type.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/ImageSharp.Tests/TestImages.cs	Added constant for extra samples test image
tests/ImageSharp.Tests/Formats/Tiff/TiffDecoderTests.cs	Added new test case for extra samples
src/ImageSharp/Formats/Tiff/TiffExtraSampleType.cs	Introduced CorelDrawUnassociatedAlphaData enum value
src/ImageSharp/Formats/Tiff/TiffDecoderOptionsParser.cs	Modified logic to parse TIFF extra sample data and patch CorelDRAW values