[3/N] Group together media-related code

[DarkLight1337]

Purpose

Improve code organization

Test Plan

Test Result

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request is a well-executed refactoring that improves code organization by grouping media-related code into a new vllm/multimodal/media package. The changes involve moving classes and functions for handling audio, image, and video from various files into this new, dedicated directory. Corresponding tests have also been relocated to a new tests/multimodal/media/ directory, maintaining a clean test structure. A notable improvement is the fix for a RecursionError that occurred when pickling MediaWithBytes objects, which is accompanied by a new regression test. Overall, these changes enhance modularity and maintainability without introducing any issues.