Support hooks on embed:"" fields (#493)

Relates to 840220c (#90)

This change adds support for hooks to be called on fields
that are tagged with `embed:""`.

### Use case

If a command has several subcommands,
many (but not all) of which need the same external resource,
this allows defining the flag-level inputs for that resource centrally,
and then using `embed:""` in any command that needs that resource.

For example, imagine:

```go
type githubClientProvider struct {
    Token string `name:"github-token" env:"GITHUB_TOKEN"`
    URL   string `name:"github-url" env:"GITHUB_URL"`
}

func (g *githubClientProvider) BeforeApply(kctx *kong.Context) error {
  return kctx.BindToProvider(func() (*github.Client, error) {
    return github.NewClient(...), nil
  })
}
```

Then, any command that needs GitHub client will add this field,
any other resource providers it needs,
and add parameters to its `Run` method to accept those resources:

```go
type listUsersCmd struct {
    GitHub githubClientProvider `embed:""`
    S3     s3ClientProvider     `embed:""`
}

func (l *listUsersCmd) Run(gh *github.Client, s3 *s3.Client) error {
    ...
}
```

### Alternatives

It is possible to do the same today if the `*Provider` struct above
is actually a Go embed instead of a Kong embed, *and* it is exported.

```
type GitHubClientProvider struct{ ... }

type listUsersCmd struct {
    GithubClientProvider
    S3ClientProvider
}
```

The difference is whether the struct defining the flags
is required to be exported or not.

kong_test.go

@@ -2413,9 +2413,19 @@ func (e *EmbeddedCallback) AfterApply() error {
 	return nil
 }
 
+type taggedEmbeddedCallback struct {
+	Tagged bool
+}
+
+func (e *taggedEmbeddedCallback) AfterApply() error {
+	e.Tagged = true
+	return nil
+}
+
 type EmbeddedRoot struct {
 	EmbeddedCallback
-	Root bool
+	Tagged taggedEmbeddedCallback `embed:""`
+	Root   bool
 }
 
 func (e *EmbeddedRoot) AfterApply() error {
@@ -2432,6 +2442,9 @@ func TestEmbeddedCallbacks(t *testing.T) {
 		EmbeddedCallback: EmbeddedCallback{
 			Embedded: true,
 		},
+		Tagged: taggedEmbeddedCallback{
+			Tagged: true,
+		},
 		Root: true,
 	}
 	assert.Equal(t, expected, actual)