[FEATURE] Integrate with Dropbox as source

[badmonster0]

We have native support to integrate with Google Drive, similarly we would love to support Dropbox.

Here is an example project to read from GoogleDrive

To Add new source integration, here is all the related code

Steps:

1. Update Rust code:
   • Add source op, example
   • Register it in the engine, example: https://github.com/cocoindex-io/cocoindex/blob/801ae8f421611e9f1359cd2e4158cc161019a104/src/ops/registration.rs#L10
2. Add a source spec in Python, example: https://github.com/cocoindex-io/cocoindex/blob/555d3286c02f4f7887145418dbd866bfcb91d243/python/cocoindex/sources.py#L21-L28
3. Add a new example to test. You can follow the example of gdrive_text_embedding for Google Drive.
4. Add the new source to the documentation, similar to GoogleDrive.