refactor: embedded text processing modules (#2535)
This PR is similar to ocr module refactoring PR -
https://github.com/Unstructured-IO/unstructured/pull/2492.
### Summary
- refactor "embedded text extraction" related modules to use decorator -
`@requires_dependencies` on functions that require external libraries
and import those libraries inside those functions instead of on module
level.
- add missing test cases for `pdf_image_utils.py` module to improve
average test coverage
### Testing
CI should pass.
