Enable blocking pull requests that add new warnings

[jonasblunck]

Things to consider for feedback:

1. Invoking the check for warnings as the last thing in RunPipeline.ps1. Could move to a step in _BuildALGoProject.yaml, but not sure how value it adds?
2. Should the setting be named 'checkForNewWarnings', or 'blockNewWarnings', etc?
3. We could optimize runtime by only comparing warnings when the number of warnings from baseline has changed, with the risk of ignoring new warnings if introduced at the same time as other warnings are removed.
4. Warnings are only stored in the log if the setting 'enableCodeCop' is set to true. Should we throw an error if checkForNewWarnings is $true but enableCodeCop is $false?
5. A new compiler version could add new warnings. In this case, a new 'official' build has to run before a PR can be completed.