Support CancellationToken with async methods

[angelaki]

Some async methods (especially most of the IRazorEngineTemplate methods) lack of an CancellationToken parameter. There are probably cancelable jobs under the hood I guess?

[adoconnection]

Not really, this is where all the magic happens and its all sync.
In my understanding if calling task was cancelled with token this code will continue execution anyway.

RazorEngineCore/RazorEngineCore/RazorEngine.cs

Lines 64 to 111 in 64f3b34

	RazorProjectEngine engine = RazorProjectEngine.Create(
	RazorConfiguration.Default,
	RazorProjectFileSystem.Create(projectPath),
	(builder) =>
	{
	builder.SetNamespace(options.TemplateNamespace);
	options.ProjectEngineBuilder?.Invoke(builder);
	});
	RazorSourceDocument document = RazorSourceDocument.Create(templateSource, fileName);
	RazorCodeDocument codeDocument = engine.Process(
	document,
	null,
	new List<RazorSourceDocument>(),
	new List<TagHelperDescriptor>());
	RazorCSharpDocument razorCSharpDocument = codeDocument.GetCSharpDocument();
	SyntaxTree syntaxTree = CSharpSyntaxTree.ParseText(razorCSharpDocument.GeneratedCode, cancellationToken: cancellationToken);
	CSharpCompilation compilation = CSharpCompilation.Create(
	fileName,
	new[]
	{
	syntaxTree
	},
	options.ReferencedAssemblies
	.Select(ass =>
	{
	#if NETSTANDARD2_0
	return MetadataReference.CreateFromFile(ass.Location);
	#else
	unsafe
	{
	ass.TryGetRawMetadata(out byte* blob, out int length);
	ModuleMetadata moduleMetadata = ModuleMetadata.CreateFromMetadata((IntPtr)blob, length);
	AssemblyMetadata assemblyMetadata = AssemblyMetadata.Create(moduleMetadata);
	PortableExecutableReference metadataReference = assemblyMetadata.GetReference();
	return metadataReference;
	}
	#endif
	})
	.Concat(options.MetadataReferences)
	.ToList(),
	new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary)
	.WithOptimizationLevel(OptimizationLevel.Release)
	.WithOverflowChecks(true));