Allow sharing of private files

[roman-khimov]

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

I'm always frustrated when I follow the default path and then can't solve some trivial problem. So I can create a "private" container (which is the default) and then I would like to share it with my friend, which is rather typical. How would I do that?

Describe the solution you'd like

Several possibilities:

• special "share" link with embedded Bearer allowing to read for anyone who has this link
• more convenient eACL rule manager allowing to specify an address to share object with (hi, Account-based EACL targets neofs-api#278 though, likely not really possible at this stage)
• a combination of those?

[roman-khimov]

See https://github.com/nspcc-dev/neofs-s3-gw/blob/master/docs/authmate.md#generate-presigned-url and https://docs.aws.amazon.com/AmazonS3/latest/userguide/using-presigned-url.html also.