[admin-api] download re-encrypted files from the inbox

[jbygdell]

Please describe the feature

As a submission manager, I want to be able to download (metadata) files from the inbox so they can be validated.

Acceptance criteria

• Re-encrypted files are downloadable
• Documentation is updated
• Tests verifying the changes are added

Additional context

Queries like the one below could be used to download a file.

This one uses a custom header but adheres to the semantics by using GET request

curl -H "Authorization: Bearer $token" 
-H "Public-Key: LS0tLS1CRUdJTiBQVUJMSUMgS0VZLS0tLS0KTUlJQklqQU5CZ2txaGtpRzl3MEJBUUVGQUFPQ0FROEFNSUlCQ2dLQ0FRRUE0Ym1ZOFQ1amhsQ0Z0UzRjY3luOApaRGZ0V2syN3lPY1BnS2Jhazk5MS9tMXJzMzQ3NFJLSFlqYzlHd1gwSUNTYVVOMDJmQ2RFRQUIKLS0tLS1FTkQgUFVCTElDIEtFWS0tLS0tCg==" 
-X GET https://HOSTNAME/users/SUBMITTER_NAME/file/FILEID

Estimation of size

small/medium/big

Estimation of priority

low/medium/high