[FEATURE] Add custom folder path option for Smithy-generated handlers

### Describe the feature

Add an option in the Smithy handler trait to define a custom folder path for the generated handers.

### Use Case

Current settings generate all handlers inside the `src` and `test` folders with a flat hierarchy. Need an option to customize the individual handler path.

### Proposed Solution

Add a new optional `subdir` option to the Smithy handler trait?

### Other Information

We may also need to add this option for async handlers.

### Acknowledgements

- [X] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### PDK version used

0.23.66

### What languages will this feature affect?

Typescript, Java, Python

### Environment details (OS name and version, etc.)

Linux, Windows etc

Locate the Smithy handler trait and the generator paths for TypeScript, Java, and Python, then inspect how synchronous and asynchronous handlers choose their output folders. Confirm the intended optional subdir behavior across src and test outputs; done means each affected language can generate handlers under the requested custom path without changing the default layout.

Written by the indexing model from the issue text.