forward statistics from STAC raster:bands

[vincentsarago]

ref #571

This PR Does:

• extend AssetInfo with metadata and dataset_statistics
• forward the statistics within the ImageData (when not provided by the image itself)

To Do

• [x] The metadata is not forwarded to the ImageData object when we create it from a list of ImageData https://github.com/cogeotiff/rio-tiler/blob/d0f9d11894f3b8a5d2e02fc637393873ddce0a57/rio_tiler/models.py#L458-L485