plugin/cancel: add context cancelation plugin (#2711)

* plugin/cancel: add context cancelation plugin

Per review comments on #2704, move this into a plugin that gets called.
Add the most minimal plugin, tests and documenation.

Signed-off-by: Miek Gieben <miek@miek.nl>

* plugin/cache: add timeout option

review feedback: add option to set custom timeout.

Signed-off-by: Miek Gieben <miek@miek.nl>

* spelling

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/done.go

@@ -0,0 +1,14 @@
+package plugin
+
+import "context"
+
+// Done is a non-blocking function that returns true if the context has been canceled.
+func Done(ctx context.Context) bool {
+	select {
+	case <-ctx.Done():
+		return true
+	default:
+		return false
+	}
+	return false
+}