Python: adding external tokenizer to python text_chunker

[gramhagen]

Motivation and Context

addressing issue #1387
chunking text should allow use of an external tokenizer

Description

added pass-through of an token counting function, defaulting to the existing _token_counter() method
while fixing a type hint bug I got sucked into making a few changes to clean up the code.

future work would be nice to add chunk overlap functionality similar to langchain's TextSplitter

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[shawncal]

@gramhagen Cool change! Thanks for the contribution.

Welcome to Semantic Kernel!