Improve publish pipeline and nuget packing

Our publish pipeline and nuget packing logic can use some irmpovements.

Agents for publish pipline

We often see:
This agent request is not running because you have reached the maximum number of requests that can run for parallelism type 'Microsoft-Hosted Private'. Current position in queue: 42
because the microsoft subscription has a limmited set of agents for this

Nuspec location

The nuspec files are in a single folder in vnext\scripts. It would be more manageable if these were next to the actual sources and part of the project file and part of the actual built bits so it is part of the Artifact..

Nuget publish logic

We currently have a flag to deploy shared layout files. This should be moved out to an individual step

Publish should be trigged by CI and use the binary outputs

Currently publish re builds all binaries. It would be nice of the CI could take care of that...
Complication is the beachball bump and beachball publish.... Ideall we separate out the git .patch, .tgz npm packages and upload those to artifacts and have publish push them.
For nuget we should unpack, sign contents and repack...

Deal better with noCI case.

Start by reading the publish pipeline configuration and the NuGet specification files under vnext\scripts. Trace how binaries, npm packages, patch files, and shared layout files are currently built and published, then define completion around CI-produced artifacts, separate publish steps, NuGet repacking, and improved noCI handling.