Correct Shovel struct field types

[pathcl]

DestinationURI/SourceURI it's an slice at least in rabbitmq 3.8.9

issue related: #103

[michaelklishin]

@pathcl thank you. How can I reproduce the issue this strives to address?

[michaelklishin]

A really basic Shovel in 3.8 uses "singular" values for source and destination URIs:

[
  {
    "node": "rabbit@warp10",
    // ...
    "name": "amqp://",
    "vhost": "/",
    "type": "dynamic",
    "state": "running",
    "src_uri": "amqp://",
    "src_protocol": "amqp091",
    "dest_protocol": "amqp091",
    "dest_uri": "amqp://",
    "src_queue": "src-queue",
    "dest_queue": "dest-queue"
  }
]

[michaelklishin]

I now realize the difference: the above is from the Shovel management HTTP API endpoint and #171 uses the runtime parameters endpoint, which indeed can use slightly different values. In addition, there can be multiple URIs (that are tried until one of the succeeds).

[pathcl]

@michaelklishin I also noticed if we use passwords like '#123' rabbit will complain with 400 error since we're not urlencoding

[michaelklishin]

Percent-encoding of various parts of URIs is a separate issue. Some would argue that libraries should not do that at all.
We already percent-encode hostnames (all HTTP API clients do) but I can't think of a client that does that to Shovel URIs.

[michaelklishin]

With this PR, tests cannot compile because they provide a single URI (which is still perfectly valid) and not an array.

We have to support both types for these fields. I'm looking into it.