Support for additional SignTool flags

[ben-wallis]

Cake.Common.Tools.SignTool is currently missing support for several flags relating to certificate locations.

All of these should be optional, I'd propose the following additional properties are added to the SignToolSignSettings class:

signtool sign flag	Property Name	Type
/ac	AdditionalCertificatePath	string
/s	CertStoreName	string
/sm	UseMachineStore	bool

Additionally, the TimeStampUri property of SignToolSettings is currently mandatory due to the following code in SignToolSignRunner.cs:

if (settings.TimeStampUri == null)
{
	const string format = "{0}: Timestamp server URL is required but not specified.";
	var message = string.Format(CultureInfo.InvariantCulture, format, GetToolName());
	throw new CakeException(message);
}

Using a timestamp server should be completely optional as it significantly slows down the signing process and is often unnecessary for things such as internal release builds.

Also, it's worth noting that it's not possible to use ProcessArgumentBuilder's Append() or Prepend() methods to add any parameters to SignTool either, as Prepend() inserts them before the "sign" parameter of "signtool.exe sign", and Append() inserts them after the list of files to sign. Currently we're using reflection to insert into the private _tokens list within ProcessArgument which is obviously not ideal.

[MihaMarkic]

Fully agree, I added a PR for /sm argument

[MihaMarkic]

@ben-wallis There is a workaround, as suggest by Joseph Musser: create a new instance of ProcessArgumentBuilder, merge arguments and return it as result.

[devlead]

Fixed by #2437