Add example values for schema properties.

The OpenAPI spec allows specifying "example" values for properties in the data model

{
  "type": "object",
  "properties": {
    "id": {
      "type": "integer",
      "format": "int64"
    },
    "name": {
      "type": "string"
    }
  },
  "required": [
    "name"
  ],
  "example": {
    "name": "Puma",
    "id": 1
  }
}

https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md...

We should provide this when we can. It will make the example model better. In OpenAPI docs this would also make the "Try it out" feature actually work. Presumably this would be useful for other tools that would use the OpenAPI output.

Examples

Entity references

On node schema "type" is reference to Content type entity.
For any entity reference that references a config entity type like this we could get a sample entity and add the id as example.

Text formats

For text fields we could use a valid text format id for "format"
Is there any pattern we could use here for other field types?