Support outputting a list of databases as JSON

[kate-goldenring]

Takes a similar approach as spin templates list and provides an option of listing databases as JSON

Example output:

[
  {
    "database": "brilliant-kangaroo",
    "links": [
      {
        "label": "default",
        "app": "todo"
      }
    ]
  },
  {
    "database": "mydb",
    "links": [
      {
        "label": "hello",
        "app": "todo"
      },
      {
        "label": "foo",
        "app": "bar"
      }
    ]
  }
]

[kate-goldenring]

@itowlson I noticed that in spin templates list, we hide the optionality of json output format. Is there a reason for that -- and one that is related to whether it should be hidden here too?

[itowlson]

@kate-goldenring Just to reduce clutter in the help text. In that case, the JSON is primarily expected to be consumed by other programs, not be interactive users, so I felt it was not worth making interactive users look at the flag.