vs16.11 onboard signing

[YuliiaKovalova]

Fix ##12398

Use MicroBuildSigningPlugin@4 and split pipeline on prod and exp.
Exp run: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=12265616&view=results

[dotnet-policy-service]

Hello! I noticed that you're targeting one of our servicing branches. Please consider updating the version.

[Copilot]

Pull Request Overview

This PR upgrades the MSBuild pipeline signing infrastructure to use MicroBuildSigningPlugin@4 and separates the experimental pipeline from the production pipeline to improve build management and security.

Key changes:

• Updates signing plugin from MicroBuildSigningPlugin@1 to MicroBuildSigningPlugin@4 with enhanced configuration
• Creates dedicated experimental pipeline configuration for exp/* branches
• Refactors common build job logic into a reusable template

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
azure-pipelines/.vsts-dotnet-exp-perf.yml	New experimental pipeline configuration for exp/* branches with signing validation parameter
azure-pipelines/.vsts-dotnet-build-jobs.yml	New template containing extracted common build job logic with updated MicroBuildSigningPlugin@4
.vsts-dotnet.yml	Main pipeline refactored to use new build jobs template and exclude exp/* branches

[JanProvaznik]

exp insertion has to be updated too, so it takes bits from exp build and inserts to 16.11

[YuliiaKovalova]

exp insertion has to be updated too, so it takes bits from exp build and inserts to 16.11

done