Collect unknown flags

Checklist

• Are you running the latest v3 release? The list of releases is here.
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What problem does this solve?

it's often desirable to create a wrapper around an existing command

in this case while -- is canonical (#2072), in some cases you want the usage to be transparent.

Solution description

allow and collect unknown flags

optionally, allow interspersing known an unknown flags

prior art

• in kong you can use passthrough to stop flag processing and pass all subsequent flags/args to a []string

var CLI struct {
	Flag string   `short:"f"`
	Args  []string `arg:"" optional:"" passthrough:""`
}

// use CLI.Flag

// Execute desired command
exec.Command(cmd, CLI.Args...)

mycli -f foo --uknown-flag unknown-arg

• in spf13/pflag and hence cobra, you can ignore unknown flags, but the pr to collect these flags is still open - https://github.com/spf13/pflag/pull/199

justification

i believe this is not a niche requirement, and some clis may need to accept unknown args and pass them to another program

the simplest case is time ls -a

a different case would be something like helmx --custom-flag get all

see more use cases here: https://github.com/spf13/cobra/issues/739

Describe alternatives you've considered

• using --

possibly related to #144

N.B. this is not a complete feature request, as i have not accurately defined the desired semantics, but would love for comments