Add repository / homepage URL to help_template keys

[ajeetdsouza]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

4.3.2

Describe your use case

It would be great if I could add the application homepage URL to the help page via a template key, eg.:

{before-help}<bold><underline>{name} {version}</underline></bold>
{author}
{repository}

...

An example - ripgrep uses the following format for the start of the help page:

ripgrep 13.0.0
Andrew Gallant <jamslam@gmail.com>

ripgrep (rg) recursively searches the current directory for a regex pattern.
By default, ripgrep will respect gitignore rules and automatically skip hidden
files/directories and binary files.

Use -h for short descriptions and --help for more details.

Project home page: https://github.com/BurntSushi/ripgrep

Describe the solution you'd like

A key for {homepage} and {repository} would fix this.

Alternatives, if applicable

No response

Additional Context

I want to use the repository variable in zoxide's help page, so that users can report issues more easily.

[epage]

clap can't look this information up directly. CARGO_PKG_REPOSITORY and CARGO_PKG_HOMEPAGE would need to be looked up within your code and passed in. Even with the derive API, we prefer to at least opt-in for inferred fields to be passed in (#[command(name)]). We'd need to add a Command::homepage or something for users / the derive to pass this in that would only exist for the sake of the help template which to me is questionable value for the cost that everyone would be paying.

However, we are (slowly) working on a plugin system to clap. The easiest example is that clap should know nothing about ValueHint but instead it should be in clap_complete but we allow users to attach state like that to an Arg without caring what it is. So a user could set clap_complete::ValueHint and clap_complete can read it when needed.

We could have help-template specific plugins that would make it so only the people using them would be paying the cost for them.