Allow sharing files to specific accounts

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I can't give access to to some file for a specific account. #124 is nice, but it allows anyone with a link to get access. Sometimes people want to do something more specific.

Describe the solution you'd like

Sign a token allowing to access for account X. The problem is, it won't work without nspcc-dev/neofs-api#241.