Description of issue or feature request:
Targets file hashes stored in targets metadata currently are generated ad-hoc when calling repository tool's write or writeall methods, requiring the corresponding targets file to be available at that time.

To make the tool more flexible (e.g. for PEP458) it should be possible to use previously (out-of-band) generated hashes that were already added to the roledb (see #1003).

Current behavior:
When calling repository tool's write or writeall, tuf generate hashes for all relevant target files in the roledb.

Expected behavior:
write and writeall should take an optional use_existing_fileinfo kwarg, which skips the hashing of target files in underlying functions () and instead uses the fileinfo in roledb (*), failing if not available.

(**) stacktrace is:
write/writeall --> repository_lib._generate_and_write_metadata --> repository_lib.generate_targets_metadata

(***) Depends on roledb changes through #1003