Fixed download of images with mixed layer media types

[Madeeks]

Support for zstd layers was introduced with commit 4d843dc, however the current implementation assumes that all layers in the image have the same media type (.layers[0].mediaType).

When importing images which have mixed layer types (e.g. gzip compression and zstd compression), this behavior causes layer extraction to fail, since layers with a type different from the first one are not pre-processed properly.

This PR addresses the issue by:

1. saving the manifest content into a variable instead of consuming it immediately to parse blob digests and media types
2. passing the manifest content to the docker::_download_extract function
3. having each blob extract its own media type from the manifest in the docker::_download_extract function

This is an alternative implementation to the one in #250.
This strategy removes the need to handle an array for media types in docker::_download() (e.g. parsing from manifest, pruning out entries for cached layers, more complex call to GNU parallel), at the cost of storing the image manifest content in a string variable.