Adding count_tokens for rest

[shilpakancharla]

No description provided.

[MarkDaoust]

For count tokens we should keep the examples in sync with the plan from the doc:

#445

Since they want to use the same snippets in the guides.

[MarkDaoust]

Please test your samples, this is failing:

$ bash samples/rest/count_tokens.sh 
[START tokens_text_only]
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   248    0   137  100   111   1423   1153 --:--:-- --:--:-- --:--:--  2583
[START tokens_chat]
{
  "error": {
    "code": 400,
    "message": "Invalid JSON payload received. Unexpected end of string. Expected , or } after key:value pair.\n\n^",
    "status": "INVALID_ARGUMENT"
  }
}