session: replace PToken structure with PrivateToken interface

In previous implementation PToken contained the full Token structure.
Since private token is used for data signature only, storing unused
fields of a user token is impractical. To emphasize the purpose of
the private part of the session, it makes sense to provide the user
of the session package with its interface. The interface will only provide
the functionality of data signing with private session key.

This commit:

  * removes PToken structure from session package;

  * defines PrivateToken interface of private session part;

  * adds the implementation of PrivateToken on unexported struct;

  * provides the constructor that generates session key internally.

session/types.go

@@ -1,13 +1,9 @@
 package session
 
 import (
-	"crypto/ecdsa"
-	"sync"
-
 	"github.com/nspcc-dev/neofs-api-go/internal"
 	"github.com/nspcc-dev/neofs-api-go/refs"
 	"github.com/nspcc-dev/neofs-api-go/service"
-	crypto "github.com/nspcc-dev/neofs-crypto"
 )
 
 type (
@@ -23,17 +19,20 @@ type (
 	Address = refs.Address
 	// Verb is Token_Info_Verb type alias
 	Verb = service.Token_Info_Verb
-
-	// PToken is a wrapper around Token that allows to sign data
-	// and to do thread-safe manipulations.
-	PToken struct {
-		Token
-
-		mtx        *sync.Mutex
-		PrivateKey *ecdsa.PrivateKey
-	}
 )
 
+// PrivateToken is an interface of session private part.
+type PrivateToken interface {
+	// PublicKey must return a binary representation of session public key.
+	PublicKey() []byte
+
+	// Sign must return the signature of passed data.
+	//
+	// Resulting signature must be verified by crypto.Verify function
+	// with the session public key.
+	Sign([]byte) ([]byte, error)
+}
+
 const (
 	// ErrWrongFirstEpoch is raised when passed Token contains wrong first epoch.
 	// First epoch is an epoch since token is valid
@@ -58,8 +57,3 @@ const (
 	// ErrInvalidSignature is raised when wrong signature is passed to VerificationHeader.VerifyData().
 	ErrInvalidSignature = internal.Error("invalid signature")
 )
-
-// SignData signs data with session private key.
-func (t *PToken) SignData(data []byte) ([]byte, error) {
-	return crypto.Sign(t.PrivateKey, data)
-}