Add support for fully qualified names in message defnitions

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In the Foxy Message API Review it came up that it might make sense to add support for Fully Qualified Names in the msg format so that subfields could be referenced like they are now supported for command line tools with `<package_name>/msg/

It's not entirely clear if this is worth doing as only currently messages can be embedded in other messages. Services and Actions don't make sense in that context so the syntax is not ambiguous so adding the more verbosity might not be worth it, but it will reflect the namespaces better.