Refactoring: Use enums for maintainability

[Prajna1999]

Is your feature request related to a problem?
Currently, all if-else cases rely on string-based matching, which affects maintainability. Additionally, the documentation for STT/TTS/Config needs updates.

Describe the solution you'd like

• Replace all magic strings with Pydantic enum types for improved maintainability.
• Update relevant documentation for STT/TTS/Config.

Original issue

Describe the current behavior
Currently all if-else cases do string based matching. Also update documentation for STT/TTS/Config

Describe the enhancement you'd like
Instead of strings, have pydantic enum types for better mainatainability and relevant documenation update.

Why is this enhancement needed?
Maintainability perspective.