[ci] Generate and upload a Software Bill of Materials

[pjcollins]

Context: https://eng.ms/docs/initiatives/executive-order/executive-order-requirements/executiveorderoncybersecurity/softwarebillofmaterials
Context: https://eng.ms/docs/cloud-ai-platform/devdiv/one-engineering-system-1es/1es-docs/secure-supply-chain/ado-sbom-generator
Context: https://github.com/xamarin/yaml-templates/blob/4cabafec2cec4fc02b44635b196020cd00a60f03/compliance/sbom/job.v1.yml

A "post-build" stage has been added that will run after all "build" and
"prepare release" stages. This stage consists of one job which will
generate a Software Bill of Materials using shared yaml templates.
Other post processing jobs can be added to this stage in the future if
needed.

[pjcollins]

@mjbond-msft It looks like the latest changes generated a manifest: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=5638652&view=artifacts&pathAsName=false&type=publishedArtifacts.

The .json file only contains .vsix, .pkg, and .nupkg file information, and nothing about the contents of those packages. I'm not sure if all sub-components need to be included as well, but I think you have already raised this question? We can probably wait to land this until we have some more information about the artifacts that need to be captured.

[pjcollins]

Going to use this to test out changes in https://github.com/xamarin/yaml-templates/pull/169 before merging.