NeoFS containers can be removed within a trusted session. There is a need to
take this into account during removal inspection.
Define `SessionToken` / `SetSessionToken` methods on `RemovalWitness` struct
in order to embed `session.Token` to it.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Define `RemovalWitness` structure which groups the information required to
prove and verify the removal of a container. This type is going to be used
in container Delete-related methods.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>