Security: Implement RestrictedPython for safer custom code execution

## Background

Currently, the custom code compiler uses raw `exec()` to execute user-provided Python code, which could potentially be unsafe as it has unrestricted access to Python builtins and the global namespace.

## Proposed Solution

Implement [RestrictedPython](https://restrictedpython.readthedocs.io/) to provide a safer execution environment for custom code. This will:

1. Restrict access to potentially dangerous builtins
2. Run code in an isolated namespace
3. Prevent access to sensitive operations

## References

- Original PR discussion: https://github.com/airbytehq/airbyte-python-cdk/pull/174#discussion_r1917458832

## Implementation Notes

- Add RestrictedPython as a dependency
- Replace current `exec()` implementation with RestrictedPython's secure execution
- Add tests to verify security restrictions are working as expected

## Security Considerations

This enhancement will improve the security posture of the custom code execution feature by preventing potentially malicious code from accessing sensitive operations or resources.