BuildChecks - add 'Panic' severity

[JanKrivanek]

Context

dotnet/sdk#45342 (comment)

In case of Build Checks in category of security checking - e.g. UntrustedLocationCheck - erroring out, while leaving the build running might not be enough. We might want to have ability to prevent further execution of the build.
On option is to add severity that would not only log error, but as well abort/cancel the running build.

[JanKrivanek]

This wold be most potent for the 'in-node' checks, that run within the worker node and there is no delay nor asynchronity brought by loggin infra and deliver to the main node.