Use OneBranch for signing

[florelis]

Change the release pipeline to use OneBranch instead of 1ES Pipeline Templates. This allows us to use their registration for ESRP instead of having our own.

Notable changes:

• Added new NuGet config pointing to an internal feed so we don't have a direct dependency on nuget.org
• We don't explicitly publish any artifacts. OneBranch automatically publishes anything in the ob_outputDirectory
• The signing task changes to the OneBranch wrapper, where we only need specify the cp_code

Microsoft Reviewers: Open in CodeFlow