Rework SetupNuGetSources to support WIF Service Connections

[mmitche]

Adds a set of templates which generate aad tokens in pipelines, and uses these templates in a new template which enables internal sources.

There are a few interesting aspects to this:

• Remove the use of PATs when authenticating to feeds that are in the same project. Instead, just add the feeds and call NuGetAuthenticate.
• Alter SetupNuGetSources.ps1 so that it doesn't bake a PAT into the NuGet.config. Instead, use the environment variable strategy.
• I did not change SetupNuGetSources.sh except to enable its use without a PAT (so that devs can use it on a local machine). The reason is that I wanted to avoid having to parse and generate json in bash. in addition, the powershell ecosystem is pretty robust at this point. We can use powershell in all build cases.
• Existing uses of SetupNuGetSources.ps1 should still work even without switching over from PAT usage, if they have a subsequent call to NuGetAuthenticate

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[mmitche]

I've got a couple more tests to run with this for arcade main, and I want to rerun my msbuild tests. But this should be g2g after that.

[mmitche]

Test with replaced SetupNugetSources.ps1/sh in existing infra: https://dev.azure.com/dnceng/internal/_build/results?buildId=2461793&view=results
Test with new templates: https://dev.azure.com/dnceng/internal/_build/results?buildId=2461803&view=results

Need a couple of SB tweaks

[mmitche]

Test with new SetupNuGetSources, without using the new templates except in source-build.yml: https://dev.azure.com/dnceng/internal/_build/results?buildId=2462612&view=results

[mmitche]

Hold on merging until I have places that use the script updated to include a NuGetAuthenticate call after.

[mmitche]

PRs opened.

[MilenaHristova]

@mmitche can we backport that to release/8.0?

[mmitche]

Yes, it will get backported to 8 and 6. I'll be starting that today, but I want to get a few verified usages before merging