Expose `sha256` as `selected_file_hash` in the reviewers API

[willdurand]

See: mozilla/addons-code-manager#1394 (comment)

EnTeQuAk said:

Correct, file.hash is the sha256 hash of the XPI file. To avoid confusion, I'd suggest maybe selected_file_hash in addition to selected_file which already exists?

That would indeed make some backend related code easier and we could maybe even remove the need to have sha256 inside of entries at all - given that we only show it for opened files. Does that sound reasonable?

┆Issue is synchronized with this Jira Task

[KevinMind]

Old Jira Ticket: https://mozilla-hub.atlassian.net/browse/ADDSRV-25