TokenCount performance issues (microsoft#580)

### Motivation and Context

We encountered some performance issues and implemented the following
fix.

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### Description

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We updated the TokenUtils class within the Skills Web API to address
these issues. A newly-introduced tokenizer has been implemented for
encoding text, which significantly improves the efficiency of the
TokenCount method. As a result of this change, the performance of the
token counting operations has been enhanced, ensuring more accurate and
faster responses for users.

### Contribution Checklist

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- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [Contribution
Guidelines](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

webapi/Skills/Utils/TokenUtils.cs

@@ -17,6 +17,8 @@ namespace CopilotChat.WebApi.Skills.Utils;
 /// </summary>
 public static class TokenUtils
 {
+    private static SharpToken.GptEncoding tokenizer = SharpToken.GptEncoding.GetEncoding("cl100k_base");
+
     /// <summary>
     /// Semantic dependencies of ChatSkill.
     ///  If you add a new semantic dependency, please add it here.
@@ -98,7 +100,6 @@ internal static void GetFunctionTokenUsage(SKContext result, SKContext chatConte
     /// <param name="text">The string to calculate the number of tokens in.</param>
     internal static int TokenCount(string text)
     {
-        var tokenizer = SharpToken.GptEncoding.GetEncoding("cl100k_base");
         var tokens = tokenizer.Encode(text);
         return tokens.Count;
     }