Add base classes for multi-commands.

Review Request #11882 — Created Nov. 24, 2021 and updated


We'll soon be adding two new commands, rbt api and rbt review, which
both want to implement additional subcommands (rbt api get,
rbt review publish, etc). This change adds the framework to do so,
consisting of two new base classes. SubCommand should be inherited for
each subcommand offered, and MultiCommand aggregates them together.
Arguments from the MultiCommand are added in to the subcommand

The one tricky thing here was getting --help to do the right thing. In
the case of MultiCommands, we need to actually parse the arguments in
order to get the right subparser, but doing so was enforcing any usage
requirements. It turns out that the correct fix here was to allow
add_help to default to True in the parser initialization. This
allows us to call parse_args and have it not fail.

I did also have to move the logging setup out of the initialize
method, so that we wouldn't attach multiple log handlers.

Used this extensively with the upcoming review command.

Add base classes for multi-commands.
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