ngclient: use MetaFile/TargetFile verification API when it exists #1410
Description
Related to #1377 and #1361: Once Metadata API offers the verification functionality, we should use it in ngclient and verify file lengths and hashes for both metadata and targets.
This would mean removal of helper functions like _check_file_length() _check_hashes_obj(), and removal of hash checking code in metadatabundle