Added string pooling feature

[sandrohanea]

Improve String Pooling in Whisper.net

Summary

This PR enhances the string pooling mechanism in Whisper.net to optimize memory usage and reduce allocations when handling strings, particularly in high-performance scenarios. The changes focus on refining how strings are pooled, retrieved, and returned, ensuring efficient reuse.

Changes

• Implemented StringPool to manage and reuse strings, reducing memory pressure.
• Optimized UTF8 string conversion using pooled strings to avoid unnecessary allocations.
• Introduced ReturnString to correctly reinsert strings into the pool for future reuse.
• Integrated StringPool usage across WhisperProcessor where applicable.
• Ensured thread safety and performance by structuring the pool using SortedList<int, Stack<string>>.
• Added example of how to use default string pool.

Benefits

• Lower GC Pressure: Reduces the number of string allocations, minimizing garbage collection overhead.
• Improved Performance: Reuses pre-allocated strings where possible, speeding up frequent string operations.

Notes

• This PR only modifies the string pooling logic; no other functional changes were made.
• The default behavior remains unchanged unless explicitly configured to use the string pool.

Inspired by #339