Run SignCheck on .NET Core

[ellahathaway]

Fixes dotnet/source-build#4848

This PR includes the following changes to allow SignCheck to run on .NET Core:

• Breaks SignCheck into two projects, a cli project and a task project. This is necessary to be able to keep repos that currently use the framework executable from being disrupted by the addition of .NET Core as a target framework. The cli executable is only available to run on framework, whereas the task can run on both core and framework.
• Disables authenticode verification on .NET Core. This functionality is currently unusable on .NET Core
• Adds eng/common/sdk-task.sh to run the sdk task on mac & linux.

[ellahathaway]

Testing in pipeline: https://dev.azure.com/dnceng/internal/_build/results?buildId=2631754&view=results

Updated pipeline run: https://dev.azure.com/dnceng/internal/_build/results?buildId=2634190&view=results

Errors are expected! I intentionally tested the validation on mac & linux with signed and non-signed artifacts. The SignCheck correctly marked the signed and unsigned packages.

Having issues with the file path permissions on linux and mac in the test pipeline run & need to figure out why the tests failed in this PR, but the general content of this PR is otherwise ready to be reviewed.

Fixed with b325256

Going to mark this PR as ready to review to get a review on the current state, but adding the do not merge label.

Removing the label.