`Container` and `Object` types are transmitted in single `session.Token`
message. They differ only by session context.
Share common parts of the message in `commonData` struct. Embed struct
into `Container` and `Object`. Make `ReadFromV2` methods to check
protocol compliance. Make `Unmarshal`/`UmarshalJSON` to check field
format in case of presence only.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
There is a need to duplicate session token owner, e.g. in container
created within the session. For such cases we need to have the ability
to receive session issuer.
Add `Container.Issuer` method. Transform `IssuedBy` to helper function.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
