Add custom deny attributes to access application

[Justin-Holmes]

Description

This PR adds support for custom deny attributes on Access applications.

Has your change been tested?

Example curl request:

curl --request PUT \
  --url https://api.cloudflare.com/client/v4/accounts/<ACCOUNT_ID>/access/apps/<APP_ID> \
  --header 'authorization: Bearer <TOKEN>' \
  --header 'x-auth-email: <EMAIL>' \
  --header 'x-auth-key: <AUTH_KEY>'
  --data '{
        "aud": <REDACTED>,
        "cors_headers": {
                "allow_all_headers": true,
                "allow_all_methods": true,
                "allow_all_origins": true
        },
        "created_at": "2020-10-14T21:52:27Z",
        "domain": <REDACTED>,
        "name": "test app",
        "policies": [],
        "allowed_idps": [],
        "auto_redirect_to_identity": false,
        "session_duration": "1m",
        "custom_deny_url": "https://www.cloudflare.com",
        "custom_deny_message": "asdf"
}'
{
  "result": {
    "aud": <REDACTED>,
    "cors_headers": {
      "allow_all_headers": true,
      "allow_all_methods": true,
      "allow_all_origins": true
    },
    "created_at": "2020-10-14T21:52:27Z",
    "domain": <REDACTED>,
    "name": "test app",
    "policies": [],
    "allowed_idps": [],
    "auto_redirect_to_identity": false,
    "session_duration": "1m",
    "updated_at": "2020-12-16T20:07:11Z",
    "custom_deny_message": "asdf",
    "custom_deny_url": "https://www.cloudflare.com",
    "type": "self_hosted"
  },
  "success": true,
  "errors": [],
  "messages": []
}

The Cloudflare API docs will be updated with these new fields shortly.

[jacobbednarz]

woo! this was something I've been looking for too! 🙏