Definition export: inject default queue type into virtual host metadata

[michaelklishin]

With this change, GET /api/definitions and GET /api/vhosts will inject a default queue type (DQT) into virtual host metadata when default_queue_type is set in rabbitmq.conf (v4.0.x-only) and the virtual host in question does not have DQT defined for it:

default_queue_type = quorum

Closes #12776.
Closes #12835.

[mkuratczyk]

All the parts you've worked on so far work well for me. There's one more endpoint which I think should
have the DQT: /api/definitions/vhost (eg /api/definitions/%2F). Right now it contains no metadata whatsoever right now and therefore no DQT.

Additionally, rabbitmqctl export_definitions (completely different code path unfortunately).

[michaelklishin]

@mkuratczyk ready for another round.

[mkuratczyk]

Works great, thanks. The only thing that I wasn't 100% sure about is that
if you export definitions with a DQT set and import them to a fresh node (that just defaults to classic), the DQT doesn't get set to the value from the definitions JSON.
But given that imports are additive, I think that's ok - we are not overriding an existing value.