Custom properties missing in GHRepository

[nardew]

Describe the bug
GHRepository class (interface to repos/org/repo_name GitHub endpoint) is missing custom properties. They are mentioned in the GitHub documentation at the root level as

"custom_properties": {
      "type": "object",
      "description": "The custom properties that were defined for the repository. The keys are the custom property names, and the values are the corresponding custom property values.",
      "additionalProperties": true
    }

[bitwiseman]

PR welcome.

[gitPushPuppets]

I've got a working local poc for GET /repos/{owner}/{repo}/properties/values and have started exploring the POST for create or update. @bitwiseman, do you have advise on how I should architect this? The response body of the GET and request body of the POST are slightly different and custom properties also exist at the org level. I am considering modeling my Custom Properties solution off of GHHooks.