Feature request for completing external subcommand

[shannmu]

Please complete the following tasks

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

Clap Version

master

Describe your use case

Cargo can invoke external commands. If there is a cargo-foo binary in the PATH environment variable, we can run it like a Cargo subcommand by using cargo foo.
What i want is to complete cargo fo[TAB] to cargo foo

Describe the solution you'd like

• Add a struct like SubcommadCompleter
• impl CustomCompleter for SubcommadCompleter
• invoke the complete callback when there is no positional argument on Command, or the pos_index is 1.

Additional Context

See #3166 to get more context