Validate ImageBuilder container image signature before use in pipelines

Currently, pipelines pull the ImageBuilder container image without verifying its signature:

https://github.com/dotnet/docker-tools/blob/9b94edafb9caedd3967fd5efe88a5ca270430353/eng/docker-tools/templates/steps/init-imagebuilder.yml#L34-L42

We should add a `notation verify` step after pulling the ImageBuilder image to validate that it was signed by Microsoft before using it to build and publish .NET container images.

This will require the Notation CLI, root CA certificates, and a trust policy to be available on the pipeline VM images. Ideally these would be baked into the VM images rather than acquired at pipeline run time, since that is the more secure option and the certificates are long-lived so expiration is not a concern.