Rather than a separate query, this is likely just an extension of the Supported Properties query

The initial brainstorm on #8 needed some refinement based on various UI libraries, etc. I've modified it based on feedback from @grantcodes as well as looking at UI libraries. Below the new version for discussion/iteration/destruction:

properties: [
  {
    "uid": "visibility",
    "name": "Visibility",
    "predefined": true,
    "multiple": false,
    "default": "private", 
    "options": [
      {
         "uid": "public",
         "name": "public"
      },
     {
         "uid": "private",
         "name": "private"
      },
     {
         "uid": "unlisted",
         "name": "unlisted"
      }
    ]
  },
  {
    "uid": "categories",
    "name": "Tags",
    "predefined": false,
    "multiple": true,
    "default": [],
    "optionsQuery": "categories" // This is the query sent (?q=categories) to retrieve the options
  },
  {
    "uid": "syndicate-to",
    "name": "Syndication",
    "predefined": true,
    "multiple": true,
    "default": [],
    "options": [
      {
        "uid": "uid of object",
        "name": "display name of object"
      },
      {
        "uid": "twitter-service",
        "name": "Twitter Username"
      },
     {
        "uid": "github-service",
        "name": "GitHub Username"
      },
    ]
  }
]

There are a couple notable changes and things to think through about this model.
I changed it from being a dictionary object to an array of objects with uid being the property id. This is based on looking at other Micropub queries and approaches. It's better for everything in Micropub to use similar approaches.

name is the user facing label just like the Query for Supported Vocabulary.

predefined isn't the best name, but it's the idea whether you can send ANY value or only the ones chosen by the server.

multiple is boolean because in reality a field is either going to be single choice or unlimited.

default could either be ALWAYS an array or it could mirror the multiple attribute. I kind of feel like if multiple is false, default should be a string of the uid from the options. But if it simplifies things, I'm not opposed to it always being an array and if multiple is false, then the client grabs index 0 of default.