Shovel: consider doing a passive declare before attempting to declare a queue

[michaelklishin]

See #10486 for the background.

If a dynamic Shovel is declared in a virtual host where the default queue type is not set, and the queue exists and is a quorum one, the user must specify a set of matching queue properties, or set the default queue type for the virtual host so that all declared queues by default are quorum ones.

Shovels could probe if the queue exists and if it does, just consume from them, without trying to declare them.

Note that this may be a subtle change with equally subtle "unknown unknowns."