main: add support for CLI extensions via external binaries

main: add support for CLI extensions via external binaries

This adds some logic to detect and dispatch unknown subcommands to
extensions available in `$PATH`. Additional commands can be
implemented by adding relevant `ostree-$verb` binaries to the system.

As an example, if a `/usr/bin/ostree-extcommand` extension is provided,
the execution of `ostree extcommand --help` will be dispatched to that
as `ostree-extcommand extcommand --help`.