[#283] pkg/token: Replace SessionToken into a session package

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>

pkg/session/session.go

@@ -0,0 +1,125 @@
+package session
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/pkg"
+	"github.com/nspcc-dev/neofs-api-go/pkg/owner"
+	"github.com/nspcc-dev/neofs-api-go/v2/session"
+)
+
+// Token represents NeoFS API v2-compatible
+// session token.
+type Token session.SessionToken
+
+// NewTokenFromV2 wraps session.SessionToken message structure
+// into Token.
+func NewTokenFromV2(tV2 *session.SessionToken) *Token {
+	return (*Token)(tV2)
+}
+
+// NewToken creates and returns blank Token.
+func NewToken() *Token {
+	return NewTokenFromV2(new(session.SessionToken))
+}
+
+// ToV2 converts Token to session.SessionToken message structure.
+func (t *Token) ToV2() *session.SessionToken {
+	return (*session.SessionToken)(t)
+}
+
+func (t *Token) setBodyField(setter func(*session.SessionTokenBody)) {
+	token := (*session.SessionToken)(t)
+	body := token.GetBody()
+
+	if body == nil {
+		body = new(session.SessionTokenBody)
+		token.SetBody(body)
+	}
+
+	setter(body)
+}
+
+// ID returns Token identifier.
+func (t *Token) ID() []byte {
+	return (*session.SessionToken)(t).
+		GetBody().
+		GetID()
+}
+
+// SetID sets Token identifier.
+func (t *Token) SetID(v []byte) {
+	t.setBodyField(func(body *session.SessionTokenBody) {
+		body.SetID(v)
+	})
+}
+
+// OwnerID returns Token's owner identifier.
+func (t *Token) OwnerID() *owner.ID {
+	return owner.NewIDFromV2(
+		(*session.SessionToken)(t).
+			GetBody().
+			GetOwnerID(),
+	)
+}
+
+// SetOwnerID sets Token's owner identifier.
+func (t *Token) SetOwnerID(v *owner.ID) {
+	t.setBodyField(func(body *session.SessionTokenBody) {
+		body.SetOwnerID(v.ToV2())
+	})
+}
+
+// SessionKey returns public key of the session
+// in a binary format.
+func (t *Token) SessionKey() []byte {
+	return (*session.SessionToken)(t).
+		GetBody().
+		GetSessionKey()
+}
+
+// SetSessionKey sets public key of the session
+// // in a binary format.
+func (t *Token) SetSessionKey(v []byte) {
+	t.setBodyField(func(body *session.SessionTokenBody) {
+		body.SetSessionKey(v)
+	})
+}
+
+// Signature returns Token signature.
+func (t *Token) Signature() *pkg.Signature {
+	return pkg.NewSignatureFromV2(
+		(*session.SessionToken)(t).
+			GetSignature(),
+	)
+}
+
+// Marshal marshals Token into a protobuf binary form.
+//
+// Buffer is allocated when the argument is empty.
+// Otherwise, the first buffer is used.
+func (t *Token) Marshal(bs ...[]byte) ([]byte, error) {
+	var buf []byte
+	if len(bs) > 0 {
+		buf = bs[0]
+	}
+
+	return (*session.SessionToken)(t).
+		StableMarshal(buf)
+}
+
+// Unmarshal unmarshals protobuf binary representation of Token.
+func (t *Token) Unmarshal(data []byte) error {
+	return (*session.SessionToken)(t).
+		Unmarshal(data)
+}
+
+// MarshalJSON encodes Token to protobuf JSON format.
+func (t *Token) MarshalJSON() ([]byte, error) {
+	return (*session.SessionToken)(t).
+		MarshalJSON()
+}
+
+// UnmarshalJSON decodes Token from protobuf JSON format.
+func (t *Token) UnmarshalJSON(data []byte) error {
+	return (*session.SessionToken)(t).
+		UnmarshalJSON(data)
+}