[download] Check more than file names when -continue is used

[aaperis]

Please describe the feature
As a user/admin, I want to improve the way sda-cli checks for already downloaded files so that
I can avoid situations where a file ends up being partially downloaded to a local storage.

This is an iteration on the implemented feature in #542. In connection to the discussion is that PR, we should also decide if a better name than -continue can be used.

Acceptance criteria

• enrich the filename cross-checking with more info like e.g. filesize, checksums etc
• Tests verifying the changes are added
• If deemed necessary, rename the -continue flag to something clearer

Additional context
Downloaded files are re-encrypted with the user's pubkey so encrypted filesize and checksums may differ but there should still be a way to make use at least of the filesize for better checks.

Estimation of size: medium

Estimation of priority: medium