Package Signature on CI

[etherElric]

As part of the package signing initiative, we'd like bundled packages to be prepared for upcoming constraints regarding the need to have packages with a valid signature during APV testing.

Because of this, we are going through all bundled packages' repositories creating a PR that would enable package signing on your packages.

With signing enabled, upm-ci will perform an extra step where it will send each package to a signing service and then generate the final signed artifacts.
Internally, in an attempt to ensure that everything works, we are running yamato tests where all bundled packages (including yours) are signed and validated, proving that the packages in this repository should yield no errors. The validation step is also performed in all platforms.

In the case that any error does arise, this can be simply reverted back by removing the environment variable, UPMCI_ENABLE_PACKAGE_SIGNING in the publishing jobs.

For more information, please refer to: https://confluence.unity3d.com/display/PAK/Package+Signature

You can contact us through:
#team-pkd-176-internal-package-signing
@cassandra & @FelipeMunoz

Please read the Contributing guide before making a PR.

• Read the Graphics repository & Yamato FAQ.

Checklist for PR maker

• Have you added a backport label (if needed)? For example, the need-backport-* label. After you backport the PR, the label changes to backported-*.
• Have you updated the changelog? Each package has a CHANGELOG.md file.
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR. If you do add documentation, make sure to add the relevant Graphics Docs team member as a reviewer of the PR. If you are not sure which person to add, see the Docs team contacts sheet.
• Have you added a graphic test for your PR (if needed)? When you add a new feature, or discover a bug that tests don't cover, please add a graphic test.

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[theopnv]

Running a dry-run pipeline to double check, but lgtm. https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/etherElric-patch-1/.yamato%252Fpostprocessing_publish.yml%2523postprocessing_publish_dry_run/9188592/job/pipeline