[#64] object: move package from neofs-api-go

Also, remove deprecated method.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

object/attribute.go

@@ -0,0 +1,73 @@
+package object
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/v2/object"
+)
+
+// Attribute represents v2-compatible object attribute.
+type Attribute object.Attribute
+
+// NewAttributeFromV2 wraps v2 Attribute message to Attribute.
+//
+// Nil object.Attribute converts to nil.
+func NewAttributeFromV2(aV2 *object.Attribute) *Attribute {
+	return (*Attribute)(aV2)
+}
+
+// NewAttribute creates and initializes blank Attribute.
+//
+// Works similar as NewAttributeFromV2(new(Attribute)).
+//
+// Defaults:
+// 	- key: "";
+// 	- value: "".
+func NewAttribute() *Attribute {
+	return NewAttributeFromV2(new(object.Attribute))
+}
+
+// Key returns key to the object attribute.
+func (a *Attribute) Key() string {
+	return (*object.Attribute)(a).GetKey()
+}
+
+// SetKey sets key to the object attribute.
+func (a *Attribute) SetKey(v string) {
+	(*object.Attribute)(a).SetKey(v)
+}
+
+// Value return value of the object attribute.
+func (a *Attribute) Value() string {
+	return (*object.Attribute)(a).GetValue()
+}
+
+// SetValue sets value of the object attribute.
+func (a *Attribute) SetValue(v string) {
+	(*object.Attribute)(a).SetValue(v)
+}
+
+// ToV2 converts Attribute to v2 Attribute message.
+//
+// Nil Attribute converts to nil.
+func (a *Attribute) ToV2() *object.Attribute {
+	return (*object.Attribute)(a)
+}
+
+// Marshal marshals Attribute into a protobuf binary form.
+func (a *Attribute) Marshal() ([]byte, error) {
+	return (*object.Attribute)(a).StableMarshal(nil)
+}
+
+// Unmarshal unmarshals protobuf binary representation of Attribute.
+func (a *Attribute) Unmarshal(data []byte) error {
+	return (*object.Attribute)(a).Unmarshal(data)
+}
+
+// MarshalJSON encodes Attribute to protobuf JSON format.
+func (a *Attribute) MarshalJSON() ([]byte, error) {
+	return (*object.Attribute)(a).MarshalJSON()
+}
+
+// UnmarshalJSON decodes Attribute from protobuf JSON format.
+func (a *Attribute) UnmarshalJSON(data []byte) error {
+	return (*object.Attribute)(a).UnmarshalJSON(data)
+}