Feature Request: --skip-signed option

[doxxx]

AzureSignTool has a --skip-signed option which makes it skip adding a signature to a file if it already has one, without it being treated as an error. This would be helpful in automation scenarios where some files in a directory may already be signed and should be skipped and not simply overwritten because that changes the already-signed file, which has implications for hashes of existing files etc.