[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:

     cocoindex/src/ops/registration.rs

     Line 10 in 801ae8f

     sources::google_drive::Factory.register(registry)?;

2. Add a source spec in Python, example:

   cocoindex/python/cocoindex/sources.py

   Lines 21 to 28 in 555d328

   	class GoogleDrive(op.SourceSpec):
   	"""Import data from Google Drive."""
   	_op_category = op.OpCategory.SOURCE
   	service_account_credential_path: str
   	root_folder_ids: list[str]
   	binary: bool = False

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.