feat(mux): Implement Payload storage adapter for upload field support

[jhb-dev]

Feature Request

The Mux plugin currently does not use or implement Payload's storage adapter. This limits how videos can be referenced and displayed in the admin panel.

Problem

• No upload field support: Because the plugin doesn't integrate with Payload's storage adapter, it's not possible to use "type": "upload" fields for Mux videos. Instead, a relationship field has to be used.
• No video thumbnails: Relationship fields do not support showing video thumbnails, making it difficult to distinguish between similarly named videos when selecting them.
• No polymorphic upload fields: It's not possible to use polymorphic upload fields (i.e., fields that reference multiple upload collections) with this plugin, limiting flexibility in content modeling.

Desired Behavior

It would be great if the plugin implemented Payload's storage adapter so that:

1. Mux videos can be used with "type": "upload" fields
2. Video thumbnails are displayed in the admin UI for easier identification
3. Polymorphic upload fields that combine multiple upload collections (e.g., images + videos) work as expected

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Happy to review and test a PR and help verify the implementation!