Validate MicroBuild Signing Plugin in dnceng CI Environments

We want to validate the MicroBuild signing plugin within the dnceng CI environment to proactively address any issues arising from differences in agent provisioning between devdiv and dnceng.

A new "preview" task, MicroBuildSigningPluginPreview@4, is now available for this purpose. To facilitate verification, we should create a dedicated pipeline—preferably in arcade-validation—that runs both the signing plugin and SignTool across our various CI environments. This pipeline does not need to run on a schedule, but it should remain consistent with our current dev container images.

cc @mmitche @mattnorflus