The default ShellCompDirective can be customized for a command and its subcommands

[albers]

Closes #2209, #2221

#2221 proposed a solution where the custom DefaultShellCompDirective only applied to a given command.

In https://github.com/spf13/cobra/pull/2221#discussion_r1923845125, alternative approaches were discussed. We agreed on a recursive solution, where a change applies to a command and its subcommands.

To ease review, I open a new PR for this approach.