Resolve #958

There were many breaking changes between System.CommandLine 2.0.0-beta4.22272.1 and 2.0.0. These changes:

• Update to use System.CommandLine 2.0.0 API's.
• System.CommandLine used to support Uri options, but 2.0.0 does not. Add a custom URL parser for the Azure Key Vault URL option to ensure that only absolute HTTPS URL's are accepted.
• Fix a subtle bug that may have prevented exit codes from propagating correctly. See SetAction overloads should have Task options without CancellationToken command-line-api#2562 for details.
• Disable test parallelization on SigningTestsCollection to fix test failures.
• Make minor formatting and style changes.