Support hooks on embed:"" fields

[abhinav]

(I realize that I opened this without discussing the feature first,
so if you don't think this feature makes sense, feel free to close this PR.
I've tried to outline the reasoning below.)

---

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,
allowing subcommands to declare their dependency on these resources
and use them in their Run in a highly composible manner.

For example, imagine:

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:

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.

[alecthomas]

Nice, this is awesome, thanks!