Implement `Token.Sign` method which calculates signature of the data of the
`Token` and writes the signature into it. Implement `Token.VerifySignature`
which checks if `Token` signature is presented and valid. These methods
allow to abstract the external context from the details of what kind of data
is being signed and how the signature is stored.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
There is a need to add session token to `eacl.Table` structure. To do this,
we need to replace `token.SessionToken` type to another package since `eacl`
package imports `token` one (potential cross-import).
Create `pkg/session` package and replace session token implementation to it.
Related API in `container` package is deprecated from now.
Additionally implement test generator of random session tokens.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
