Implement help system for clientv2 bridge framework.

The clientv2 bridge framework can take separately implemented command-line processing frameworks, and merge them into a single command line. At runtime, it looks at the command being executed, and determines which of its component tools should process a particular command.

This works well for everything except "help".

Implementing help consists of two parts.
(1) When "help" is run with no parameters, the bridge should generate a help message combining the help from the components;
(2) When help is run to request help for a specific command, the framework should dispatch the help request to the help command for the component that would handle the command. (eg, if CA handles commands "foo" and "bar", and CB handles commands "grok" and "grewk", then when a user executes "help grewk", the component framework should invoke "CB help grewk".