feat: support recursive injection of provider parameters

This allows provider functions to accept parameters that are injected by other
bindings or binding providers, eg. call the provider function with the root CLI
struct (which is automatically bound by Kong):

  kong.BindToProvider(func(cli *CLI) (*Injected, error) { ... })

options_test.go

@@ -89,11 +89,13 @@ func TestCallbackCustomError(t *testing.T) {
 }
 
 type bindToProviderCLI struct {
+	Filled bool `default:"true"`
 	Called bool
 	Cmd    bindToProviderCmd `cmd:""`
 }
 
 type boundThing struct {
+	Filled bool
 }
 
 type bindToProviderCmd struct{}
@@ -105,7 +107,10 @@ func (*bindToProviderCmd) Run(cli *bindToProviderCLI, b *boundThing) error {
 
 func TestBindToProvider(t *testing.T) {
 	var cli bindToProviderCLI
-	app, err := New(&cli, BindToProvider(func() (*boundThing, error) { return &boundThing{}, nil }))
+	app, err := New(&cli, BindToProvider(func(cli *bindToProviderCLI) (*boundThing, error) {
+		assert.True(t, cli.Filled, "CLI struct should have already been populated by Kong")
+		return &boundThing{Filled: cli.Filled}, nil
+	}))
 	assert.NoError(t, err)
 	ctx, err := app.Parse([]string{"cmd"})
 	assert.NoError(t, err)