feat: Allow configuring global hooks via Kong's functional options (#511)

Lets you pass `kong.WithBeforeApply` along with a function that supports dynamic bindings to register a `BeforeApply` hook without tying it directly to a node in the schema.

Co-authored-by: Sutina Wipawiwat <swipawiwat@squareup.com>

kong.go

@@ -71,6 +71,8 @@ type Kong struct {
 	postBuildOptions []Option
 	embedded         []embedded
 	dynamicCommands  []*dynamicCommand
+
+	hooks map[string][]reflect.Value
 }
 
 // New creates a new Kong parser on grammar.
@@ -84,6 +86,7 @@ func New(grammar any, options ...Option) (*Kong, error) {
 		registry:      NewRegistry().RegisterDefaults(),
 		vars:          Vars{},
 		bindings:      bindings{},
+		hooks:         make(map[string][]reflect.Value),
 		helpFormatter: DefaultHelpValueFormatter,
 		ignoreFields:  make([]*regexp.Regexp, 0),
 		flagNamer: func(s string) string {
@@ -366,7 +369,7 @@ func (k *Kong) applyHook(ctx *Context, name string) error {
 		default:
 			panic("unsupported Path")
 		}
-		for _, method := range getMethods(value, name) {
+		for _, method := range k.getMethods(value, name) {
 			binds := k.bindings.clone()
 			binds.add(ctx, trace)
 			binds.add(trace.Node().Vars().CloneWith(k.vars))
@@ -380,6 +383,16 @@ func (k *Kong) applyHook(ctx *Context, name string) error {
 	return k.applyHookToDefaultFlags(ctx, ctx.Path[0].Node(), name)
 }
 
+func (k *Kong) getMethods(value reflect.Value, name string) []reflect.Value {
+	return append(
+		// Identify callbacks by reflecting on value
+		getMethods(value, name),
+
+		// Identify callbacks that were registered with a kong.Option
+		k.hooks[name]...,
+	)
+}
+
 // Call hook on any unset flags with default values.
 func (k *Kong) applyHookToDefaultFlags(ctx *Context, node *Node, name string) error {
 	if node == nil {