Add plugin.CallWithContext method (#60)

This PR adds a new method that allows calling plugin functions with a
custom context.

This will allow developers to call extism plugins with custom timeouts
and cancellation.

README.md

@@ -89,6 +89,9 @@ $ go run main.go
 
 All exports have a simple interface of optional bytes in, and optional bytes out. This plug-in happens to take a string and return a JSON encoded string with a report of results.
 
+> **Note**: If you want to pass a custom `context.Context` when calling a plugin function, you can use the [extism.Plugin.CallWithContext](https://pkg.go.dev/github.com/extism/go-sdk#Plugin.CallWithContext) method instead.
+
+
 ### Plug-in State
 
 Plug-ins may be stateful or stateless. Plug-ins can maintain state between calls by the use of variables. Our count vowels plug-in remembers the total number of vowels it's ever counted in the "total" key in the result. You can see this by making subsequent calls to the export:

extism.go

@@ -518,11 +518,14 @@ func (plugin *Plugin) FunctionExists(name string) bool {
 
 // Call a function by name with the given input, returning the output
 func (plugin *Plugin) Call(name string, data []byte) (uint32, []byte, error) {
-	ctx := plugin.Runtime.ctx
+	return plugin.CallWithContext(plugin.Runtime.ctx, name, data)
+}
 
+// Call a function by name with the given input and context, returning the output
+func (plugin *Plugin) CallWithContext(ctx context.Context, name string, data []byte) (uint32, []byte, error) {
 	if plugin.Timeout > 0 {
 		var cancel context.CancelFunc
-		ctx, cancel = context.WithTimeout(plugin.Runtime.ctx, plugin.Timeout)
+		ctx, cancel = context.WithTimeout(ctx, plugin.Timeout)
 		defer cancel()
 	}