Specify a bounding box in `assets_download()`

[OldLipe]

assets_download() always downloads full assets. For large raster assets, users often only need data for a limited area of interest. Add support for specifying a bounding box to restrict downloads spatially.

Proposed Change
Add an optional bbox argument:

assets_download(items, asset_names = NULL, ... bbox = NULL)

• bbox: numeric vector c(xmin, ymin, xmax, ymax) (EPSG:4326, consistent with STAC).

Expected Behavior

• If bbox is NULL, keep the current behavior.
• If bbox is provided, download only the spatial subset for raster assets.
• If partial download is not supported, throw an error.
• Validate bbox length and ordering.

Acceptance Criteria

• bbox argument implemented and documented.
• Error raised when bbox-based download is not supported.
• Existing behavior unchanged when bbox is NULL.
• Tests added and passing.